It's not good to have three questions in one post. It will get very confusing when different people are answering different questions in the same thread. One question per post is better.

1. You can't deny access to a sea to anyone. But you can control it by being the only faction with ships in it.

2. Assassins get ranked higher by successful assasinations, the tougher the target the better.

3. Never payed the Hungarians befoer. Vlad huh? Ugh, don't know if I would want him on my side, but then I wouldn't want him on the other side either.