Why don't you address the hypothetical situation?
Furthermore, he is a temporal leader whose state has a 1500 year history of claiming temporal power over other sovereigns and which has never fully renounced its claims to temporal power.
What, exactly, are you referring to? Most of the quarrels between Church and state in the Middle Ages involved the state claiming illicit power over the Church, not the other way around. Regnans in excelsis? A rather special situation, involving an apparently-Catholic sovereign who was illegitimate and persecuting Catholics.
You can't wish that aspect of the problem away without abandoning the notion of Papal sovereignty.
Which isn't intrinsic to Catholic doctrine at all. The Pope's spiritual authority would be the same if he were an Italian citizen; his freedom to exercise that authority might not be.
As to the history of the Church's claims to temporal authority, I think we read the historical record so differently that discussion is fruitless.