Can someone review this garage loft design for structure and stability?
I'm not an engineer, this is just a layman review, but I think you should rethink the main supports.  I'd rather see you open the walls and support your ledger boards directly on the existing wall studs rather than lagging into them.  I don't think the 3 supports (2x4s?) under each ledger board is sufficient to prevent sheer and since you're lagging into a 2x4, that's a very narrow target to hit through the face of a 2x10 and the existing drywall.

I'd also go 10" on center for the joists, not 16.  Otherwise I'd be concerned about the span length of the 2x10's over your car.