i don't know about weight limit in michigan, but i do know, that it's 4'8" OR 8 years old. my stepson beat the height at about 7, but we kept him in the seat until he was about 8, and we had to move him out of it.
I would definatly make sure the lap belt fits the right way, otherwise, some cars have a strap you can use to make the belt come down some so it fits the shoulders properly. We use that and it really does help out a lot.
If you want to be sure, call the local police department and they can help you out with the requirements, and they might even be able to help you if you took the kids in their, and they could show you how the seatbelts should fit each kid. i know I have another car seat to put in my car, and i am heading to the police department for them to check it out and make sure it's done properly