If Roose Bolton was loyal

[Stork in the North]

How would the war have went had Roose Bolton been loyal throughout? When I say loyal I mean Greatjon-level loyalty and not out to increase his own personal power by throwing his fellow northmen into the meat grinder. I am of the popular view he did this from the get-go at the Green Fork, my evidence being the lack of Bolton banners flying in Tyrion's POV and the hail of arrows that hit northmen and westermen alike (Possibly fired from Boltons lurking in the 'Reserves'). I think that Robb could very well have won the war in 3 battles had Roose simply backed Robb completely.
-Roose addeds 3500 men to Robb's army, which is the largest contribution of any Northern lord. However, he keeps 600 men (that we know of) at the Dreadfort as a garrison. These 600 men under Ramsay are able to tear through Cassell's army of 2000 and scatter them with barely any losses, showing they were highly skilled and trained in warfare. Looking at the Dreadfort on the map I feel that 600 men is overkill for a garrison. What could possibly threaten this Castle that would warrant such a force to be stationed there? If Roose had been a Stark loyalist he would have brought an additional 500 men to Winterfell for the muster (we'll let him keep 100 at the Dreadfort just incase the Others attack or the Wall melts). This 600 strong garrison can therefore no longer be used by Ramsay, and once Theon lets him go he most likely never returns. It also seems that the majority of this garrison was mounted at Winterfell. Had Roose had an additional 500 mounted men at the Green Fork his losses might not have been so drastic, but that shall be discussed in the next point.
-Had Roose been a Stark Loyalist he would have stayed at The Twins with the Northern Foot. 18000 at Moat Cailin plus 1500 Manderlys plus 4000 Freys = 23500. Robb leaves 200 archers at Moat Cailin, 400 Freys and 400 Northerners at The Twins and takes 5000 Calvary to RR - giving Roose access to 17500 (18000 if you count the 500 Dreadfort men i mentioned earlier). Roose stays at The Twins and gives the appearance to Tywin that Robb is laying siege to the castle as they cannot press on without subduing the seemingly neutral Freys, while the Northern Calvary sneaks across the river and conducts the WW and the Battle of the Camps. Tywin only learns of WW through prisoners taken after the Green Fork, meaning if Green Fork doesnt happen yet then he doesn't learn of RR being relieved until much later. Immediately following the battle of the camps Robb sends a Raven to The Twins, telling Roose to march on Tywin, then Robb takes his Calvary South-East to approach Tywins army from behind. Roose meets Tywin head on (Tywin still may not be aware of WW and BotC) while Robb's calvary hits him from behind with a surprise attack.
-Loyal Roose doesn't force his men to do a night-march before battle, meaning that his men aren't exhausted and most certainly perform better. He would also commit a fair portion of his Dreadfort troops, who are likely to be well-equiped and trained, to the frontlines, where they can do lots of kills. It is clear upon readong GoT that he keeps his men out of this battle, as well as future battles such as Duskendale and Ruby Ford.
From Tyrions POV we learn Tywin commanded the reserve of 5000 men, who would not have the time to turn and face the 5000 strong Northern Calvary. Tywins original tactic of leaving his left open may go against him, as his reserves can't be utilised to move in and push the Stark-right into the river. Tywin's centre will be unable to Wheel and force the Stark-right into the river as Kevan will have to commit some of his forces to assist Tywins reserves. The confusion would lead to the army breaking apart. Kevan and Marbrand may be able to escape with around 4-5000 men to the SouthEast and head for KL but i see Tywin and Tyrion being captured. Having Tywin, Jaime and Tyrion in custody would mean the war is lost for the Lannisters. Robb and his army would then move to Harrenhal, prepare for an assault on KL and send for Edmure's 11,000 reinforcements from the RL - Although he could potentially bring more than 11k as with Tywin captured there is no orders given to burn the RL and Stafford Lannister also may not be ordered to raise more men at Oxcross. The Northern-Riverlands army could also wait to see how the Renly-Stannis conflict pans out while they're preparing for the KL assault. I dont see the Tyrells going solo against Stannis, Robb and Edmure so KL would fall and Joffrey would get asked to step down as king.

[Stork in the North]

Loyal Roose would have counselled Robb to not engage Tywin until he was at full force and able to position his calvary behind Tywins army. Robb himself acknowledges at Moat Cailin that Tywin has more heavy horse and a similar sized army. Therefore sending only his infantry against such a force would certainly mean defeat - loyal Roose would know this and would bring it up with Robb.
Roose's army at the GF would have 1k horse (1th of Robb's 5000 =500 as well as Roose's additional 500 who I strongly suspect are all mounted), wouldn't be tired from a night march and would have well equipped Boltons in the vanguard - I don't think this force would suffer the 7-8000 man loss that occura in GoT.

[Moriath]

I don't think Dreadfort men were that skilled as you make them out to be. Rodrik's host trusted them, which was why thetook them unawares when they started killing Rodrik's men. This led to confusion, as no one knew just what was going on and they started fighting each other. The Dreadfort men were basically the only ones who knew whom to fight, the rest just fought and killed their allies. It was chaos that led to Dreadfort victory, not their skill (which I believe was on par with the rest of the northerners)

 

Apart from that, I agree with you. Roose Bolton was a cunning and calculative man, who would've been extremely useful if he was loyal from the get go. Also, I think he would've been instrumental in pacifying the Freys if he had supported Robb.