Ya that burnt clutch makes sense with what was in the pan. I'm sure glad you didn't just change the fluid and filter and install that crispy critter. Forget a full rebuild; it was already rebuilt recently. Just check that rear band. And are you EVER going to check inside the direct drum for ring scoring? It's something you always have to check no matter what.
The drum appears to be okay no scratches or rough edges in side. The seal on the piston is a bit hard but not broken or distorted. The bands have seen there duty life cycle both front and rear.