I haven't done the event in the past (so perhaps don't take my advice too seriously), but from what I have gathered here and on youtube is that you don't want to miss the first hour or two of the park opening. With the opening show, the sunrise (a rarity at Disney parks), access to unique merchandise and snacks and really short lines for most rides (popular attractions will probably build lines quickly), and what seems like a really special "vibe", it seems like your 6:15 adr would hamper some/most/all of those things. Sorry, not trying to sound dramatic, because I am sure simply being there, plus a CP breakfast (my fav! I'll be there at 7:40am) would be great too.
I would make a decision though about the things that are most important to you and what your goals for the day are. CP breakfast? Short ride lines? 24 hr special events? Looking at CP right now, they don't seem to have later availability; however, if you are okay with leaving the event (aka if 24 hours straight in the park is not a goal), Whispering Canyon (a personal breakfast fav behind CP and Boma) has tons of availability for breakfast and is only a pleasant boat ride away. You could also look at the Poly or GF for breakfast options at around 9:30-10:30 when the park will start getting a little more crowded and you will have already taken in the morning festivities.
Also I not an expert on tickets, someone else would have to help you there. However, IF you can't make FPs for the day, those early morning hours would be prime time for rides; with 7DMT having long lines immediately and probably being ~40-60 minutes at its slowest during the event, I would make the morning rush with everyone else and tackle that, maybe pay attention to wait times and hit up another big attraction (or two) and add a few short wait ones, head out for a late breakfast outside the park, come back and enjoy less popular rides during the day and finally hitting up the popular rides again in the early morning.
This is not to say you couldn't keep the ADR at CP and still have fun though! Just depends on your goals for the day...
Our goal is to stay in Magic Kingdom for all 24 hours! (we will see how well that goes). I talked to a CM earlier today and she said that even if I am not let in before 6, just show them my dinning reservation and they will let me in right at 6 so I can get back to CP. I am going to try and look for a later ADR but if I don't get one I am just going to see where the day takes me. I have accepted that most of the day will be standing, waiting in lines (which lines at disney are still better than lots of space anywhere else) so I figure if I approach it with a hakuna matata approach I will enjoy it so much more instead of trying to stress myself out getting rides in before the crowds build. Thank you so much for all your advice!! (We almost booked the polynesian for breakfast but I am pitching a fit about staying in the park for 24 hours....My inner princess is already coming out)