Hmmm...

First, good to see you! You could always be counted on for heavy philosophy, and informing as heathens when the religionists were up to mischief.

Second, I seem to recall that a friend in your real life had said something that was making you question whether your compatibilism was valid. Obviously, that post is long gone now.

You also make a good point that we need to define "free will". The libertarian version is completely unworkable, as you say (though you didn't attach that name to it in your post). I assume that is what you meant by "four sided triangle".

...even though the agent making the choice couldn't have chosen otherwise in exactly identical circumstances.

I had assumed your friend had scored a hit on this point. For me, what you describe is not free will because "[the agent] couldn't have chosen otherwise in exactly identical circumstances". Nothing free about that, IMHO. Though you appear to be saying I am remembering that conversation incorrectly since you indicate that you weren't questioning your position.