I saw once in a circus show a man without arms playing guitar with his feet. Very amazing!!! BTW, Is it true that all violinist can play by ear?

On any string instrument without frets, you need to have the skills to play by ear!

In fact I can remember I played once with a bassist with a fretless neck where I could not resist to tell him this " shall I put some frets on your bass neck" (he really got mad )

Anyway I play by ear as well and learned to play keyboards in the past by transposing guitar chords to the keys to figure out the chords on a piano.
This was 30 years ago.
Since the arranger keyboards arrived, this now seems to be a clever way.
Due to this I play chords with three/four or more fingers/thumb which allows a lot of variation.

The playing by ear does not limit me though for getting copyright on my songs as because of Midi (which I record in realtime) the written notes/staff are ready in one go.

Nevertheless I wish I learned to read music good when I was young.

great topic though.