Transmission issues under warranty

2000 vw jetta auto. Had coolant leak, lacked power, overheated- mechanic rebuilt transmission- car still under warranty- still leaking coolant was told by a shop doing oil change that coolant leaking at t-stat housing-should mechanic have seen this and replaced it? Before rebuilt tranny it lacked power which it still does a lot its like the rpm's are rev'ing but the cars not going fast, after mechanic rebuilt the tranny the only light that comes on and stays on is round O he said someone must have clipped senors to brakes( which were checked not true) car still heats up and leakage from t-stat housing (told by shop doing oil change) should the mechanic who rebuilt tranny noticed t-stat housing leaked then and repaired it? He's now telling me they only wk on transmissions not motors. If thats the case should he have told me yes we will rebuild tranny but then you need to take it for leaking t-stat housing? Help! Do I have options? What are they please! Thank you in advance