My semi-educated guesses:
1) Yes. Whether anyone would care is another story.
2) Probably relatively well. As I recall, it was an anti-German sort of thing, rather than anti-foreign - so a Pole or Lithuanian foreigner is probably acceptable. Once the passions and irrational parts settle out, that is.
Excellent. Now, just a few more questions (not specifically for you, but before I start writing in earnest:
1. I don't like the name Sigismund for a Czech ruler (especially since the Hussites are fighting against a man with that name), so what would be an good name for Sigismund Korybut (I decided on him, since Jogila would probably be too busy dealing with the Poles and Lithuanians to gain the crown)? I'm looking at Ottokar (since Ottokar II is only two centuries previous), Weneclaus, and Borivoj, but I'm willing to look at other names?
2. If Ziska lives, will he be loyal to the foreign king? He seems to have been a rather independent fellow.
3. Could Emperor Sigismund get extra help from groups opposed to the Poles/Lithuanians, like the Teutonic Knights and others? Besides the Teutons, and the Emperor, who would have much to gain from attacking the Hussites?
4. Could traveling Hussite preachers act as "fifth columns" in neighboring lands?
5. What would happen if Emperor Sigismund died on the field of battle against the Hussites? What if he was captured?
6. Regarding in-fighting among the Hussites, could some of the more radical sects seek to carve out their own state either in Bohemia or nearby lands?
7. Finally, what happens to the major Czech nobles who stay Catholic? I don't know what happened to them, were they executed by the Hussites, or did they flee to the Catholic powers? Could some of them lead an invasion army to regain their lost lands?