Some background in my project:

1. I need a study, so I am considering moving the laundry into garage. And convert the current laundry room into a study
2. 3 car garage is attached to the house but with no direct access.
3. The part of house adjacent to garage is a guest room.
4. The garage is finished with painted wall and vaulted ceiling.
5. A dedicated split AC is in place just for the garage.
6. I am in Palm Springs, CA, so very hot in the summer and somewhat chilly in winter evenings.
7. The current split AC isn't strong enough in the summer due to uninsulated garage doors and the vents. An AC so no heat.

My thoughts:

1. Convert from 3 car to 2 cars garage.
2. Use the space for 3rd car into laundry, hobby and storage area.
3. Open a door from the guest room to garage.

Worries:

1. I shouldn't connect the central AC/Heat to the garage, should I?
2. If I put both return and supply of the central system, wouldn't the return suck in car exhaust and distribute all over the house? If I just put supply and no return, would that throw the system out of balance?
3. There are some vents about 12" above floor, I guess that is for ventilation. Would it be safe or up to code if I seal those?
4. How to insulate the garage doors?

Thanks for helping.