Welcome and sorry you are here. This may seem a bit harsh, but I'm not know for sugar coating things, I call em like I see em. So I am sorry if this hurts. But you deserve more than accepting something that most likely isn't true, and being a second choice.
I would also like to say Thank You for your service, which allows me to live my life the way I choose.
I see so many red flags in your post that I'm questioning my visual acuity. She tells you nothing happened, and you don't have proof, so you belieive her. Did you make her go get tested for STD's? Did you ask her to do this? She had way too much free time to not have met up with some of these guys, and if she met up with them it wasnt for coffee, sorry. I think you are just getting the tip of the proverbial ice berg here.
Yes you have every right to tell her to stop communicating with an old boyfriend. She is addicted to the attention these sites and the old boyfriend give her, and until you stop that she is cheating. Unless of course you are ok with it, and she is open about it, which I doubt you are.
She does not seem sorry at all, she had done nothing to change herself or work on the reasons for why she was doing it, what is sorry for is getting caught and she stopped it, but I'd bet my lunch that she if finding some other way to "get her fix". Have you followed her to school? Have you checked up on her?
I know you want to believe all is good, and she is trying but frankly I am not seeing it. Did she get a compensation package from her work of just flat out fired? If she did there should be paper work that will account for the reason of letting her go. The company has to document it, to determine unemployment eligibility.
It sucks to be in this position, and I agree that something isn't right, I would urge you to go into detective mode, and get ready to play some real hardball, because I think this is going to get a lot worse before it gets better.
I am sorry but you will find a wealth of support and knowledge here. Keep asking questions, keep posting. Stay strong.