Sonoff can overheat if overloaded (like any electrical device)…
Not really up to the job of this kind of load.
You wouldn’t want to use three or four sonoff to switch a three-phase machine, because you want all phase switching positively linked together.
It’s possible to use a single-phase sonoff to control a three-phase contactor.
I doubt the a/c unit would start up properly just by banging the mains power on though?
Yes, I assume that it has some buttons, so it might be better to wire any ESP controller (Sonoff for example) to the buttons. Then you don’t have to deal with high voltage, etc…