I’m imagining that a stronger Lancastrian succession keeps the Yorks from taking the throne, as Humphrey likely isn’t charged/framed with treason without the Eleanor Cobham debacle. His remarriage to a cousin of the Queen’s keeps everyone close without tying him to any domestic allies that could spark anything, and Henry VI’s insanity is handled better here, with York’s being brought into the fold via a marriage between Gloucester’s son and his daughter. By the time Edward of Westminster is old enough to take over his father’s Regency, everyone is a lot less suspicious of everything and Henry VI formally steps down during a lucid moment.Very interesting! Does Henry V keep the throne then? Since I think Arthur of Lancaster would be chosen over Henry Tudor, but you have him as a Duke not a King.
Alice’s marriage to the Holy Roman Emperor also strengthens the Lancastrian front, and her daughter’s marriage to Edward of Westminster is (I imagine) one of the last things Henry, Duke of Lancaster is able to get through, although it’s carried out by York. Tudor may not even marry Margaret Beaufort here, with a stronger legitimate Lancastrian succession.