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.
In a broad interpretation, the prohibitions on rent/lease/lend/sell of "the Apple Software" would apply to the whole system (harware+software), which would forbid me to sell my old Mac. This would never hold up in court. But in a narrow reading, where the prohibitions only apply to the software, separated from the hardware it does not look like there is anything stopping me from renting out (access to) the entire Mac to a single user[1], for whatever purpose and duration I want. So those 24hr and "development purposes" would be moot.
[1] There's probably more than enough precedence to uphold number-of-users restrictions of software, e.g. Windows Server CALs.