If I had a 7:45 ADR at CM's, I would be out at the MK bus stop by 6:45. Those buses run less often then the usual buses, so if you just miss one, you could have a 20+ min wait for the next one. You may get lucky and get a driver who will drop you at the CR before going to MK, but more often than not, they will just take you to MK. That's what I always plan on.
The walk from MK to CR is less than 10 mins...the sidewalk is easy to find...just head to the park entrance, and turn to the right, with the monorail overhead, buses to the right, MK to the left. Follow the sidewlk to CR and go up to CM's.
Yes, you may arrive at the CR early, but better early than late. I always leave an hour from point A to point B that early in the morning...especially when taking the bus to MK and then heading to the CR.