1. Let's say that Player A dominates Library zone with 8 tokens and Players B and C join up both with 5 tokens (equals 10) to contest him and then win the challange. So who rules the zone when B and C won the contest but A has the most tokens in the zone?
Depends on who is the contender. If, say, player B is the contender with 5 influence and C is helping him out with 5 more influence and they win the contest then player B becomes the zone's controller. A's 8 influence remain on the board since they were already there (only bidden tokens are removed).
When C is lending help put his 5 tokens under B's 5 to create a 10 stack and simplify things - this is as if B would've bidden 10 influence by himself.
2. When a player loses a zone does he also lose the prestige points he gained with it?
Exactly. Only keep in mind that the Total Prestige (City Prestige + Personal Prestige) is not cumulative turn after turn, but rather is recounted and reset each Resolution Phase.
You may want to check out my player aid to help you out during the game. The rulebook is a pain to use.