The only thing I can add is that it sounds like the torque converter (front pump) seal is leaking.
If so, I don’t see a fluid change, pan drop, or even a flush as causing this seal to leak.
If the gasket that was used on the pan was of the cork variety it’s quite possible that gasket could be leaking. I refuse to use a cork gasket for this very reason; it’s strictly rubber or cellulose.
The reason they tell you the car is undriveable is because with a steady transmission fluid drip it would be very easy for the car owner to dismiss it and keep on motoring. This could lead to a fried transmission very shortly and finger pointing over who did what or who was going to be at fault for whatever.
As to what you’re being told by the mechanic about “being angry if this was his car” and they would “take responsibility for this leak if they had changed the fluid” I can only say - utter BS on their part.
They’re simply trying to one-up the previous shops and portray them in a bad light; and believe me, this is not that rare a thing to occur.
I bet Transman knows what I’m talking about! 