Second, I wonder on what legal basis they can actually impose usage restrictions of a whole Mac (hardware+software) via an EULA as long as I don't breach any copyright (which I don't think I do if I rent out usage of the entire system for a few hours).
AFAICT (again, not a lawyer), everything rests on section 1.J.[1] "Except as expressly permitted in Section 3, you may not rent, lease, lend, sell, redistribute or sublicense the Apple Software." being enforceable. (Section 3. contains the infamous terms for Leasing for Permitted Developer Services with the 24hr restriction). I don't know if such terms would hold up in court. If I read the thing correctly, the the EULA forbids people from lending their MacBook to a friend, or even resell it, which strikes me as quite absurd.