Were Roose’s actions during the Battle of the Green Fork normal (or at least, not unheard of) from a medieval perspective? A nobleman taking advantage of a battle to weaken their fellow nobles?

It’s more that the time gap between the Green Fork and his full-scale betrayal at the Red Wedding is unusually long. During the Wars of the Roses, there were quite a few people who betrayed their own side during a battle, but usually they defected right afterwards. To quote myself:

• “perfidious” Lord Stanley, the Earl of Derby (a good pick for historical parallels to Walder Frey), who came from a staunch Lancastrian House, but was married into the Yorkists through the Earl of Warwick. At the Battle of Blore Heath, one of the opening battles of the war, Stanley raised 2,000 men at his King’s command but then withheld them just a few miles away as a Lancastrian army was defeated by a smaller Yorkist force. When Edward IV took up the Yorkist cause,  Stanley defected and fought alongside the new King; when Warwick defected from Edward IV, Stanley fought to restore Henry VI for the last time. Remarkably, he managed to get appointed to Edward IV’s royal council even after his betrayal. He then married Margaret Beaufort, the mother of Henry Tudor, while helping Richard III fight the Scots. Famously, Stanley held back his forces at Bosworth Field despite Richard III holding his son hostage, and then charged Richard’s rear once the King was fully committed, personally crowning Henry VII to make sure he ended up on the right side.
• Andrew Trollope, a career soldier, was one of Warwick’s closest lieutenants from his service at Warwick’s side in Calais. After sailing to England with Warwick’s fleet, Trollope defected to the Lancastrians with his entire force at Ludlow. His tactical genius proved invaluable to Margaret D’Anjou, as he devised the stratagems of sending false reinforcements to York wearing looted surcoats to lure him out of Sandal Castle, and in hiding the two wings of the Lancastrian forces in the woods to destroy the Yorkist army at Wakefield, and was knighted at the Lancastrian triumph of the Second Battle of St. Albans. Less skilled at “re-ratting” than Lord Stanley, Andrew Trollope died on the field at Towton.
• Lord Grey, a veteran of the wars in Acquitaine, served the Lancastrian cause consistently…right up until the Battle of Northampton, when he ordered his men to lay down their arms as Warwick’s forces neared the fortifications of the Lancastrian left flank, after which Warwick rolled up the line, sent the Lancastrian army into a panicked rout, and captured King Henry VI. For his pains, Lord Grey was awarded a disputed manor, the position of Lord Treasurer, and with the hand of Joan Woodville, the title of Earl of Kent. Apparently the wages of treason are real estate.
• William Neville, the uncle of the Kingmaker and Baron of Fauconberg, fought by Richard Duke of York’s side in France for seventeen years. After being ransomed by the King and then paid a thousand pounds in restitution, Neville fought for the Lancastrians at the First Battle of St. Albans, but then managed to get appointed to the Royal Council during Richard’s reign as Lord Protector; when the wars started up again, Neville defected to the Lancastians again with perfect timing, right before the disastrous Battle of Wakefield. When Edward IV came along, Neville fought for the new King, and got made Lieutenant of the North, Lord Admiral, and the Earl of Kent (Lord Grey would take the title after his death in 1463).”