You can take the monorail to MK from CR, but you travel all of the way around to the other monorail resorts before you get to MK. On the return from MK to CR (BLT) you can take the resort monorail and it's the first stop.
It's real easy to get around BLT with a wheel chair, all of the hallways are wide. Just make sure you request a handicap room with a roll in shower (if that's what you want). The bed will be lower. The room is supposed to allow the movement of a wheelchair as well. They also have handicap rooms that do not have a roll in shower if you prefer. The smaller slide at BLT is listed as an ADA approved slide. Don't know if your DH would be interested in it or could even use it. I know young children love it.
As a family you can easily go up to the TOTW Lounge for fireworks viewing. However, if you prefer no crowds and if you don't have an MK view, you can see the fireworks from the elevator lobbies. I'd pick one above the 7th floor or the 15th floor would even be better (to see over the monorail easily) and use the north elevators. They have a more direct view of the the castle and the fireworks.
The parks should be less busy after New Years than either Thanksgiving week or Christmas week. However, NYE is very crowded. Maybe more so that the other two holidays.