Here some answers:
1) First, I think I understand how ties are resolved when determining the order for Corruption, but want to be certain. Page 5 of the English rulebook states "Equalities are resolved with the turn order determined by the First Player Marker"; does this mean that players earliest in the turn order will also be earliest in case of ties for Corruption?
Yes, when several players are on the same spot in the Briber ladder, the player who is the earlier form the First player is the first to play the corruption. The equalities are broken by the turn order.
2) May I corrupt a character whose ladder I can't move further up because I'm already on its last space?
Yes you can.
3) May a player choose an altar to make a sacrifice on that an opponent already occupies? The phrase that makes me suspect the answer may be "no" is on page 7 of the rulebook and states "A player can replace his own sacrifice if he respects these rules."
You can do both: replace a opponent's sacrifice on an altar, or replace your own sacrifice on an altar.
Hope that helps :-)