Depends on when in 1455- if it's after the First Battle of St Albans then it's too late for peace and the Wars of the Roses have already begun in earnest. The younger generation of Beauforts, Percies and Clifford's will be out for vengeance, and York, Salisbury and Neville have openly deified the king, led an attack on army that had the king in attendance, fought against an army bearing the royal standard- all acts which aren't easily forgiven.
If Marg is then out of action for several weeks/a few months (confinement was a thing, yeah? How long did it last?) in the latter half of 1455 then maybe York et al can capitalise on their victory at St Albans better- IOTL he didn't achieve much and lost the ascendancy rather quickly. Conversely, if they're not coalescing around a politically active Marg, then the younger generation of Lancastrian lords could be doing God knows what and causing trouble.