I don't think free/paid/donationware matters. There are plenty of both free and paid utilities around for working with AirPods, but they're not from "Appel".

Noncommercial use can be one factor in copyright fair use. (And it's a factor in a defense. The other party can still take you to court.)

This seems IANAL to be, reverse engineering aside, to be more about trademark. I agree with the other comments that cease and desist now is the best path to hopefully avoid expensive legal bills.

Apparently there is a company called Appel, Inc.

https://www.bbb.org/us/fl/port-saint-lucie/profile/accountin...

Of course, they're not in software or hardware, so your point stands.