Since it's the first seating of the day, you really don't have to get there 15 minutes before (and in fact, they may not even be checking people in until about 5 minutes before since they're not actually open yet.) They don't fully book all the tables right at opening...they "stagger" book tables so will only book & seat about 1/4 of the tables at 8, then some more at 8:10, and some more at 8:20 and so on. Our experience has been they let you into the park about 10 to 20 minutes before the 8:00 a.m. reservations - partly depending on how far you have to walk to get to the restaurant. At HS in May, we were let in about 10 minutes til 8, so just had time to walk to the restaurant (H & V) from the gate...we got there just a couple of minutes before 8:00. But once we got there, they were not even taking names at the check-in because the computer was "down" (they didn't even check them off the paper list - just told people to wait a couple of minutes!) We were not seated until almost 8:15. We did not want to leave the area to take pictures because they had not actually checked us in or handed out buzzers, so to get seated you had to be there in the actual line. Was not a good experience at all.
At Magic Kingdom, they let us in about 7:40 so there was time to take nearly-empty pictures with the castle in the background (not completely empty-there was quite a few people heading to BB Boutique & Castle breakfast-but emptier than during regular park hours for sure.)
I would expect AK to have similar admission times--show up 20 minutes before your reservation.
Have never taken the character breakfast busses, but was told there is a character breakfast bus which picks up about an hour before first seating (like at 7:00 a.m.?) Ask at your resort. I was told the bus actually says "character breakfast" instead of park name, but can't speak from personal experience because we always drive ourselves.
