For 1, I think the FAA would look at who is PIC (who logged the flight). if the renter (PPL) logged it (I'm also assuming something like a 172), and they paid, no biggy. if the commercial pilot logged it, and the PPL paid, then it'd be illegal. If both logged it, well, then you've got other problems
I don't really think it's that hard. Who procured the aircraft? Who's flying the aircraft? If they're the same person/company, you've got issues. If not, you're good to go. I know some people have tried setting up two companies, with the wife's company as owner of the airplane, and the husband's company as employer of the pilots, but that doesn't fly (pardon the pun). Don't try to cheat the system, and you'll be ok.
As for scenario 2, that'd be legal because flight instruction is specifically exempted in 119.1, whereas the op's question is not. Now, getting the aircraft operator and insurance to agree to that scenario, that's another issue.