i’m not as excellent in close reading like u are so can I ask how long you think Roose Bolton’s ambitions to take the North have been in play? Was it always a thing you think? Did he not like the Starks or Ned personally or was it just pure ambition? How far could his Frey alliance reasonably go back?

This is something I’m still trying to work out. And it’s a little difficult to pin down, because some of his actions can be explained in other ways. For example, his throwing the Battle of the Green Fork could simply be explained as trying to increase his regional power by ensuring that his neighbors (the Karstarks, Hornwoods, Manderlys, and Cerwyns) take as many casualties as possible while he guards his own manpower. Notably, you don’t see Bolton men taking part in the fighting at the Green Fork, and indeed, almost his entire force makes it safely back to the North. 

In terms of the Frey alliance, between the Battle of the Green Fork and the capture of Harrenhal, Roose spent a lot of time camped out near the Twins with his army under minimal supervision. So somewhere between Tyrion VIII of AGOT and Catelyn V of ACOK (when we learn that Roose has taken the Ruby Ford and has married Walda Frey), an alliance is made between the two Houses. 

Certainly, I think we can say that Roose had made his alliance with Tywin Lannister around the time that he gives the orders for the disastrous attack on Duskendale in Arya X of ACOK, because Duskendale in retrospect is clearly a “shaping operation” for the Red Wedding, eliminating the Stark loyalists in his own ranks so that Roose and Walder’s forces outnumber Robb’s forces at the Twins, and because Duskendale only becomes a disaster because Roose sends word to Tywin telling him where and when to ambush the Northern infantry. 

This also makes sense, because it’s only when Roose has taken Harrenhal that he has access to his own ravenry without oversight from anyone else. Remember, up until the Battle of the Fords, Robb Stark has 400 men stationed at the Twins – with those men there, sending ravens to Tywin Lannister would be incredibly risky.