a Greek King

Alright interesting. Say despot Thomas does have legitimate surviving descendants until the time of Greek independence who remain in the public eye so to speak, could they be put forward?

Not really. Nobody was exactly clamoring to put an Austrian archduke - who theoretically had the best claim through his Gonzaga ancestry - on the throne of Greece either in Othon or Georgios' stead.
 
Even if said descendants are male line descendants and have helped lead Greece independence war?

Time and distance. Thomas' descendants weren't living in Greece AFAIK (and that was back then, what incentive would they have to move back there now?). That said, Thomas apparently did still have legitimate issue as of the 19th century, however, the descent (Princes Isaac (b.1806) and Abraham (b.1812) living in England) had been excluded from the Byzantine succession due to the fact that they had at some point (Prince Manuel (b.1455) married a Turkish princess and whose sons, Ioannes and Andreas, (from whom their descent derived) converted to Islam. So I'm pretty sure the Greeks are gonna be going 'este mi kakos?' in why they should accept that claimant (Isaac or Abraham) over some other European royal who actually means something.
 
Very true. I suppose one would need to butterfly that

Well, AFAIK, the reasons for why Andreas returned to Constantinople and threw himself on the sultan's mercy was because no one in Europe was seriously considering helping him (so to him, at the time, he appeared to have no other option: I mean, its worth noting that both Maximilian II and Karl VI recognised men as titular emperor of Byzantium (or whatever title they claimed) who descended through a bastard line from Prince Theodoros (younger brother of Emperor Ioannes VIII).
 
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