Yes, the tech can screw it up.

Besides worry and blather to the SA so he has to make up stuff to calm you down, what will you do? Would you change dealerships if there was a bigger one with better trained staff???? Would you demand to know who is doing the work and how many of these he has done?

Get EVERYTHING in writing...It is your car, you can tell them what they can and cant do- you could insist they generate a work order to only open and diagnose and report to you. In any event, you want everything in writing, that way, if something happens later, you have written documents describing what was done/touched/opened

Did he say HOW they ruled out the other possible sources that DONT require tranny removal? A second opinion at a different dealer is an option...

my 2 cents