I would also make an ADR for somewhere else (one of the MK resorts or inside the MK). I'd be pretty surprised if you made it into the MK before 7:30 pm.
I'd allow about 60-75 minutes for dinner. It is a buffet, so you can choose to eat fast it you want. After that, you'll need to catch a bus from BC to MK. The ride is about 15-20 minutes, but the general rule of thumb is to allow about 45 minutes for transportation (just in case you just miss the bus). So in the ideal schedule (60 minute dinner, get to the bus as it just pulls up) will get you to the MK at 7:00 pm. You'll need to get through bag check and then into the park. I think it's more likely that you'll get into MK somewhere between 7:30 pm and 8:00 pm if you keep the 5:40 pm ADR.