Wait...
They did the Red Wedding before Winterfell was taken by Theon?
Yeah, no, Walder and Roose would never be that stupid. The Red Wedding was only effective because afterwards, there were seemingly no Starks for the North and Riverlands to rally behind. And Robb's cause was seen as doomed, because he had lost the North with Winterfell and Moat Cailin taken. In this case though, Roose hasn't been bleeding of the Stark forces, Walder hasn't been provoked, and the North hadn't fallen.

If Robb isn't King of the Riverlands, then why is he still in the Riverlands?
This is one of many things that leads me to despise the show. They completely dropped the fact that Robb is King of the Trident as well, otherwise, he should've just turned back North, fortified Moat Cailin, and told Joffrey, Renly (and later Stannis) to go fuck themselves, and also be back in time for the harvest.

Edmure was a complete wreck after his dad died. I highly doubt that he would be doing things like arranging a marriage, which he had put zero thought to prior to his dad dying. It's also kinda a waste on Edmure's part, as he could be used to secure alliances with other houses, and notably not piss off the Freys, who already should be quite content with Robb marrying Elissa.

Wait, so you aren't rushing the pregnancy, and then immediately jumped forward to the end of ACOK...
Because by this point, Elissa should either have given birth, or should be about to.
Also, Catelyn freed Jaime before she reunited with Robb, and Robb likely would already know that Renly has died before Catelyn told him.

Events are being a bit too rearranged again for my liking.
They should already know that Renly has been crowned. Keep in mind, the meeting Robb called with the other Northern Lords and Riverlords, wasn't just in response to the fact that they had gotten word of Ned Stark's death, but also Renly being crowned.
Again, they should be in Riverrun at this point, and Robb would almost certainly leave Elissa there (funnily enough, they have a Maester there).
Robb also certainly cannot spare 500 men for Catelyn's escort. In the books he only sent 30, while in the show, I don't think it's even mentioned.
Also, I'm going to go over where things should stand at this point.
In the East, Stannis has like 7k men on Dragonstone and pretty much the only navy that's in play at this point (he also hasn't crowned himself yet). Roose's host is reforming at the Mouth of the Neck, and numbers about 10k. Tywin and his host of around 17-20k has taken up residence in Harrenhal. And to the South, Renly has an army of 100k that is slowly marching up the Roseroad. Robb is chilling at Riverrun with about 6-7k men, and his strength is dwindling after Edmure allowed his lords to take their men home. To the west, Stafford Lannister has is assembling a new host (that numbers like 10k), 3 days ride from Lannisport, while Forley Prester has the remnants of Jaime's host at the Golden Tooth, numbering 3-4k.
Robb also doesn't depart Riverrun for a bit after this either. Robb at this point is actually in a really tough spot as he doesn't have the strength to take on Tywin at Harrenhal.
I'm fine with bending cannon, I just need a reason for bending it.
Also, where's the Blackfish, Jason Mallister, Tytos Blackwood, Jonos Bracken, Karyl Vance, Edmure, Maege Mormont and Galbart Glover? I do like Roose Bolton's absence though.

Heat Stroke is kinda a big deal (also, it's not too terribly warm at this point, in both book and show, snow is observed at the Twins, so it should hardly be warm enough to get it).
Also the sequence of events here is a little off.
1. Robb mentions using Jaime to exchange for Ned, but in the last chapter, we got word of his execution.
2. If they are referencing queens, it should be Cersei and Margaery Tyrell (who is Renly's queen).
3. Robb was crowned in Riverrun. He wasn't just crowned King in the North, but King of the Trident as well, with the Riverlords bending the knee to him. He wasn't crowned in some camp. The reason I bring this up is that you've mentioned the fact that they were breaking the siege of Riverrun twice, so at this point, Robb should be in the castle.

Also, Robb is 15 at this point, not 18.

Sorry that I mostly use my reviews to point out continuity errors and nitpick. Overall I am enjoying this.

I really cannot tell if this is meant to be show or book cannon.
You mention breaking the siege of Riverrun, which was pretty much ignored by the show, yet have the same plans as in the show (sidenote: Robb would've already split his forces at this point).
In the show, Robb's battle plans are absolutely insane and make no sense.
In the books, prior to making it to the Twins, Robb already had the Blackfish set to work screening his movements, and they shot down all ravens to let Tywin know that Robb had parked his army outside the Twins for a few days.
After making his deal with Walder, Robb split his host in two. He took 5k men, all mounted (he got an additional 1k men from the Mallisters as well), west to break the siege of Riverrun, while Roose a host of 17k, which consisted of all of their infantry, and 1/10th of the cavalry, south along the Kingsroad (kinda the exact opposite of what they did in the show).
Robb's ruse worked because he had already been screening his movements, and Tywin didn't know exactly how many men Robb had prior to him making it to the Twins, and how much he had afterwards, (and even then, Robb prior to crossing only had about 18k, plus like 1-2k freeriders, and Roose continued south with a host of 17k, which from a scouting perspective isn't all that noticeable).
He certainly did not send 2k men to pretend to be 20k, and if he had, the entire ruse would've failed. Plus, Roose and his host served as a buffer as previously mentioned, both to the Twins, but also Moat Cailin and the Neck as well. Roose wasn't specifically instructed to meet with Tywin, but was mostly there to make sure nobody suspect Robb had actually split his army.
Prior to the Battle of the Whispering Wood, Jaime had been personally leading men to deal with the scattered remnants of Edmure's host who were raiding his camps. So Robb and the Blackfish exploited this, having about 200 men, feign a raid on the Northern Camp, which Jaime was stationed at and personally commanding, and they lured Jaime into the Whispering Wood. Jaime only anticipating having to deal with a few hundred men only brought 2k men (all the cavalry in the Northern Camp, while the Eastern Camp had none), outnumbered 3 to 1 and caught off-guard, Jaime's host was destroyed, with most of his men slain or captured. Robb and his men lost only like 200 men in a battle where they took out a force of 2,000.
Then, Robb and his forces descended on the Northern and Eastern Camps in what was known as the Battle of the Camps (when one is laying siege to Riverrun, you have to split your army in three, and they are cut off by rivers), and took them out, freeing the Riverlord hostages (like Edmure), and fully breaking the siege. In the southern camp, the 1k freeriders, defected to Robb's side (under the command of a mysterious Tyroshi sellsword who is never mentioned again), while the other 2k archers and infantry were scattered and reformed at the Golden Tooth. Robb also faced minimal casualties in this battle as well, and actually likely gained more men from the freeriders defecting than he lost between the two battles.
Sorry for my rant about the show, and how they oversimplified things way too much. Also, if Robb hadn't sent the army to distract Tywin, there's no point in doing so now, considering that they would probably be battling in a few days anyways, and he's just sending 2k men away for literally no reason (they never would make it to Tywin's host in time).
Also, Robb received word of Ned's death after the Battle of the Whispering Wood, in both book and show, and would also have gotten word of Renly crowning himself as well at this point.

Robb isn't crossing the Twins to go South though...
He's crossing to go West.
If he wanted to, he could continue south without ever talking to Walder, the problem was that Robb wanted to split his forces, with him taking the cavalry west to go break the siege of Riverrun, while Roose would take the infantry, and a small portion of the cavalry south along the Kingsroad to distract Tywin. Robb needed to cross at the Twins because the river had flooded it's banks, so Robb couldn't make his own crossing.

Additionally, it's worth considering why Robb didn't marry one of Walder's daughters.
1. Robb is 15, so is still technically a minor in Westerosi custom. Yes, there are cases of people younger than 16 getting married, but those aren't too terribly common.
2. Robb didn't have the time. He needed to cross ASAP.
3. The wedding would be an extremely rushed and quiet affair. They can't have word of Robb making a deal with the Freys get to Tywin or Jaime, because otherwise, they might see through Robb's ruse. They certainly would not have a feast.
4. It gave Walder an opt out. Setting the terms to have Robb marry one of Walder's daughters or granddaughters after the war is great for him. Assuming Robb wins his war, this means that he's likely one of the most powerful men in all of Westeros, and now Walder is able to marry one of his descendants to him. If not, then Walder can opt out, and he's not tied to Robb permanently.

Finally, taking Elissa west is kinda a dumb move. Robb isn't too desperate for an heir at this point, as Bran and Rickon are safe at Winterfell, so it's not like he needs to keep trying for one, which means there's no reason to take her west. Plus, the destination that Robb is heading to, Riverrun, is under siege. They are bringing her into a war zone, when victory is far from certain, which puts her in pretty significant danger. The Twins on the other hand are a secure position as it is well fortified, and is currently garrisoned with over 800 men (and Castles are force multipliers), and with Robb's army of about 5k (6k once they pick up the Mallisters), and Roose's army of 17k, serving as buffers to the Twins against Jaime's host of 15k (with likely quiet a lot of men being left behind to maintain the siege, and Tywin's host of 22k (not that there's much reason for Tywin or Jaime to assault the Twins either to be completely honest). Plus, unless Elissa is a good rider, she could probably end up causing them to waste precious time that Robb is trying to make up for at this point. Robb can just send for her once they've freed Riverrun, which would allow her to be safely brought there (else they could just be needlessly giving Jaime and Tywin another hostage). Or better yet, she can just stay at the Twins, since at this point, Robb's strategy is pretty much get enough hostages and bleed the Lannisters to force them to make peace, and secure Ned's release, which would mean that Robb would return to the Twins in not too long. And keep in mind, Catelyn would probably advocate for Elissa remaining at the Twins given that she actually wed Ned under remarkably similar circumstances.

This story was amazing. I loved every bit of it, even when you had me in tears. When you had Elissa thinking that Robb might be unfaithful and then switched it to Jon and Daenerys my heart stopped for a moment...until I saw Jon's name. This is by far one of the best GOT fanfics I have read so far and I cannot wait to read more of your stories. :)

Robb has a point what he did to that soldier that Tywin suborned was a vengeance killing (murder to give it its' proper name) that is very different to Elissa killing an enemy soldier in the middle of the Red Wedding (what she did was to kill someone who was attacking her family in battle). Elissa's victim was an armed man on the battlefield the cowardly traitor Robb butchered was chained and helpless.

Still I don't think anyone in RiverRun will object (and I don't think they would even if Robb were not the King); the traitor dissevered to die and in the cold blooded political sense it will not hurt a bit for everyone to know that if you push Robb too far (by attacking a member of his family directly) he will be as savage as a Wolf and slaughter those responsible.

This story was so incredible. I read it in a day. I couldn’t put it down. I can’t even imagine all of the work put into writing it. You crawled inside so many characters’ heads and still managed to give each one their own distinctive personality that didn’t become muddled throughout. Your characters were so fantastic. Your OCs fit the story and the world so perfectly. You kept each of George R. R. Martin’s characters in character, but you created such wonderful growth in them. This story was a delight to read. You should be so very proud. Thank you so much for sharing.

A* STORY HUN XXXX

"He is just like you, but for the eyes," she told him then as she shifted the baby into his arms."

I'm sorry but what? When elissa says that it means he looks exactly like Robb apart from having his eyes. But then robb goes onto say Stark eyes. It makes no sense. How can she say that then Robb say the opposite. Think you need to reword it.

AW HAPPY FAMILIES PREFECT STORYXxxx

I don't think you realise that sex doesn't actually happen as much as you write. But whatever