OHHHHH,,BUMMER!! Your mechanic admitted to not changing that one freeze plug. They always look good....from the outside.
You not only have to pull the transmission,,,youll have to remove the starter,flywheel, and bell housing to get to that freez plug. Part of the floorboard may be too high to get to it so youll also have to,(maybe) drop the rear of the engine down to be able to get to it. May be easier to pull the engine back out. Sorry for the bad news.
Good luck,
