I suppose you could arrange things in a different order, or have the Lancastrians victorious at Tewkesbury but Prince Edward still killed. Henry VI would then still remain as the returned king, but without an heir and no likelihood of one. Since charges of bastardy against Edward IV seem to have predated Richard III, its possible that these could be used. The problem is that if Henry survives as king, Edward and his children take refuge in Burgundy, with the potential of leading an invasion out of Ireland perhaps. Clarence would be in a position as a late ally of Warwick to be adopted as Henry's heir
Best Regards
Grey Wolf