But is there any chance that one rental company would actually be able to guarantee CarPlay availability (meaning no GM vehicles)? Seems unlikely to me.
Sure, but they can't guarantee that you'll get a non-GM car, since they never know if they'll have availability in advance. Sometimes you show up and they don't have the vehicle type, or any vehicle whatsoever.
And I would pick Enterprise over Avis if it came down to availability of CarPlay.