I'd suggest making Mina the 'Spare' character because she has one card revealed anyway, plus she is generally used as a DracuSonar.
It is a tiny extra advantage, but this is a game you play for the experience, rather than the player balance.
I agree. (I haven't played 3rd edition with less than 5 players, but it's not that different from 2nd edition.)
Running two hunters gives that player more to keep track of, so they're more likely to make mistakes, which helps even things out. It also means fewer brains to outwit Dracula. That isn't really significant with 4 players, but with 3 and even more so with 2, it can make a big difference.
Anyway, Dracula is very strong in 3rd ed so making the hunters a bit stronger is no problem at all!