When buying an extra charge move slot, is it possible for the pokemon to gain its legacy move? Or using a charge Tm specifically on the second charge move?
"You can use a charge TM on either move."
More specifically, when you use the charged TM, the game asks you which move you want to replace. Choose wisely, if you accidentally fat finger a legacy move, it's gone, forever. There should be a confirm screen before doing that, but there currently isn't.