I think you've blown ALOT of silly things way out of proportion. You have such a suspicious mind!
Ok so maybe some thing dont quite add up but you seem to read into them alot, i think alot of its in your mind.
She's young, she wants to have fun with her friends....but that doesn't mean she cant be with you aswell! And the things shes told you about her past..well thats exactly what it is, the PAST. You seem to think she is the same, would do the same things now...well people change, and as she said herself , she would never do that stuff being attached.
As for who to believe...your friend or her...well have you actually sat down and discussed this properly? tell her what your friends say, rationally ask her what she thinks...at the end of the day, your in a relationship with HER, not your mates. You said yourself these are RUMOURS passed around from person to person. theres nothing to be afraid of by talking to her about it. To some extent thats the key to a relationship, talking...discussing..
Your girlfriend seems like a normal young woman, and i think if you carry on in this jealous, posessive and paranoid way you'll end up pushing her away.
Thing like saying you dont approve of her friend etc is not a good idea...its going to seem like you dont want her to have friends.
i think you just need to take one big chill pill. Enjoy your time NOW, not what happened in the past, listen to your girlfriend...talk to her...spend time together. dont worry about what anyone else says...