A Neville-Percy match makes sense. The two families hated each other - and one (Percy) went with the Lancastrians and one (Nevilles) with the Yorkists, but it was the Percy's tardiness at Bosworth where they were going to turn out for Richard III against
a Lancastrian claimant, that partially decided that battle IIRC. Have to check my sources, though @BlueFlowwer
or @Philippe le Bel
might be more up to date on that. - but marriages in those days were generally between two families who hated each other's guts and expected the spouses to get along, even though they'd been raised to hate one another, as a way of temporarily papering over differences so they could deal with other business.
Partially. The Nevilles cleaved to the Yorkists for quite a long time, until Edward IV made a few decisions with his other
head and married Elizabeth Woodville, in effect humiliating Richard Neville (16th Earl of Warwick, the Kingmaker, father of Richard III's wife Anne). This, the decreased favor shown to Neville, and the increasing power entrusted to and respect shown toward Earl Rivers (Elizabeth Woodville's brother) all led to Richard Neville's support of rebellion, and to his directly joining Henry VI in his last attempt to regain England.
With Nevilles disgraced and out of the picture, especially once Richard III's wife Anne Neville died, the Percys were able to rather comfortably switch sides.
BUT as for marriages, it really depended. Nations that were centuries-long enemies such as France and England had many royal marriages (Edward I and Margaret, Edward II and the famous Isabelle the She-Wolf, Henry V and Catherine, the planned marriage between Princes (later Kings) John or Richard and Alix (sister of Philippe II), among others). But then there were many duchies and counties that regularly married other nearby families all out of amity, and were close friends and allies. Other times marriage was between the girl of a noble, prestigious family and a less noble man who had attained wealth and status, and could thereby benefit her family immensely. It all depended really, even within the royal family.
Within just this time frame you have:
Catherine of Valois (forced to marry Henry V by peace treaty, because he destroyed France on the battlefield)
Margaret of Anjou (unclear as to why, certainly she had a forceful but loyal personality, perhaps could give England some sort of alliance to the south of France)
Elizabeth Woodville (literally a love match, Edward IV fucked it all up here)
Anne Neville (first married to Edward of Middleham son of Henry VI to tie Richard Neville to the Lancastrians after his defection, later fought over by future Richard III and George of Clarence because of her huge inheritance from her now dead father)
Elizabeth of York (most senior Yorkist heir according to many people's thinking, her blood able to legitimize and stabilize the shaky rule of Henry VII who barely has a leg to stand on)
The reasons really all depend on the situation, more often than not they are matches of convenience between families who are ambivalent or even slightly friendly, not hateful. But one had to tread carefully around royals anyway.