1) Are there any new laws that apply from MCUJ that didn't apply before hand? (Obviously do not do drugs in any way), but if you have friends that are officers, are you now bound to not fraternize with them?
You're BDCP so you're basically a civilian with an ID card. Fraternization doesn't even exist for you until you put on that uniform at OCS. Please do not start referring to your officer buddies as "sir". In fact, take this time to grow a full beard. Even better if you can get your active duty ID card with a full beard in the nastiest T-Shirt you own. Don't go memorizing the UCMJ. You already pretty well summed up what you can't do with the drugs thing.
Realize that you might be on the hook to buy a commercial ticket home if something happened and couldn't get a space-A flight back in time.
I wouldn't really think that that would be an issue for BDCP. At least at my recruiting district they really didn't have anything too pressing for me to do, so unless you're actually running out of leave days, I wouldn't anticipate problems getting it extended. Just make sure you don't cut it too close.