[xmodem]

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".

[ghaff]

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.

[lioeters]

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.