Sometimes multi wire circuits are run in mixed wire/breaker sizes around here. One example would be a bathroom. The #14 for lighting and #12 for GFI. Myself I try to keep equipment off of lighting. I'm sure some would not like this multi wire set-up. Maybe someone could figure the math on this with extreme conditions. I doubt it would make much differance doing it this way. Never had a complaint or seen a problem. I try to tape the network together and put the breakers next to each other to avoid confusion for someone elce but thats just my thing.
No trick question. Just never did that trick before. Can't see any hazard and the reason he wanted to do it was so he can put the plugs on a 20a. circuit and the lights on a 15a. circuit. I couldn't find anything in the code that prohibited it. Thought you code aficionados knew of an article in the book that would apply. As for the definition of garage:
GARAGE n 1: an outbuilding (or part of a building) for housing automobiles 2: a repair shop where cars and trucks are serviced and repaired [syn: service department] v : keep or store in a garage; "we don't garage our car"