There is a chance that she'll tell you she's been experimenting. Of course, that's cheating also.
You asked about boundaries....would you let her bring her bf over? Then her lesbian lovers aren't allowed over either. I guess you'll just have to wait and see what she says. I guess you should consider what you think about worst case scenario..she's cheating on you with OW. What is your stance? I don't recommend that it be you are fine with it, but don't fly into a rage either. I'd probably shoot for being calm and collected, but stern and ready to kick her butt out. Hopefully for you infidelity is a boundary that you don't tolerate her crossing. But that's cart before the horse. It would be unusual for a spouse to admit an affair...but just wanted you to prepare yourself mentally.
Having the other talks, especially about details surrounding child support, etc, is perfectly permissible. Just handle it calmly and rationally, like a business transaction, and keep emotions out of it.
Lastly, yes, get your own checking account. Time to separate out finances and cancel joint credit cards. It's also reasonable to expect that she won't go out every night and party. Personally, I wouldn't say anything about it, but document every night she goes out and when she comes home. It will show who has been the responsible caregiver should it come down to who gets physical custody.
Just some food for thought
You cannot be lonely if you like the person you're alone with. Dr. Wayne Dyer