Could of Buckingham's rebellion of been successful if he had a senior male royal, who could of immediately been declared King upon the Princes in the Towers deaths? It might of stemmed the tide of desertions and attracted a few lancastrians? The rebellion might of been a lot stronger with a secondary plan of John II instead of Henry Tudor marrying Elizabeth of York and becoming King.
I think that depends on what Buckingham's motives are- and they're murky as hell. I mean, they has to be some overarching ambition here- Richard had rewarded him very lavishly and he was essentially the king's right hand man-
if said ambition was to attain the throne himself, using the princes as a pretext, then having a fellow adult male rebel with a superior claim is unwelcome.
If it was a matter of genuinely wanting to liberate/restore the princes, and then when he finds out they're dead he desperately casted around for another claimant and eventually settled on Tudor, then supporting John is reasonable because he's a better candidate than Henry.
Whether it's
successful probably depends on how personable and competent John is and whether he can actually hold an army together long enough to actually face the king in the field. Though if there is a battle you'd expect the older, more militarily experienced Richard to be successful (though that was true of Bosworth as well).
The reason he [Henry] didn't have constant rebellions was because he was married the senior Yorkish heiress.
Look, I agree that marrying Elizabeth did smooth over Henry's assumption of power and gain him the support (or tolerance) of many Yorkists (particularly the Woodvilles and their affinity).
But he was still massively insecure- and he
did face constant rebellions and plots. There's Lambert Simnel and Stoke; Perkin Warbeck, who launched multiple rebellions and was a massive nuisance for the entirety of the 1490s; Richard de la Pole was making abortive invasion plans with his buddy Francis I as late as
1523.
At least initially, these were far from impossible and had the support of notable figures like Lincoln and Thomas Fitzgerald (brother to the Earl of Kildare) in Simnel's rebellion; Viscount Lovell and the Stafford brothers rebelled in 1486; William Stanley, who had (probably) saved Henry by his intervention at Bosworth was executed in 1495; Maurice Fitzgerald Earl of Desmond supported Warbeck at the siege of Waterford in 1495(?); James IV invaded (ostensibly, he was self-interested) in Warbeck's favour in 1497; you had Margaret of York Dowager Duchess of Burgundy supporting
anyone challenging Tudor. Some of these, arguably, were no less likely to succeed than Tudor's own invasion in 1485.
The fact that people (ostensibly) bought and plotted on behalf of the impostors shows that Elizabeth wasn't necessarily considered the Yorkist heir[ess]. So, Henry's marriage to Elizabeth certainly didn't prevent Yorkist plotting and he
did face several rebellions- hell, if Henry VIII believed
he was the legitimate Yorkist heir through his mother he wouldn't of ruthlessly persecuted the Pole family whose only crime was being friendly with Mary and being related to the exiled Reginald (indeed Henry went after
anyone with Plantagenet blood- Buckingham in 1520, Edmund de la Pole in 1513). Beheading a harmless 67-year old who had committed no crime is hardly the action of someone who was secure in their belief in their own right to the throne.