1997 Chevy Blazer Transmission Fluid Change / Now Transmiss

I took my car in for an oil change at a Quick Lube place, and they had a special to change transmission fluid. I knew I needed this done because my mechanic in Grand Rapids (I recently moved to Detroit area) had told me a while back I needed this done. After the service, I noticed it was running differently. When I accelerated from stop, it was very sluggish and it wasn't shifting gears. Then it stopped driving altogether (found out the fluid had leaked out). I brought it back and he said his mechanic made 3 errors: Pinched the gasket, didn't use silicone and screws were too tight. He redid the service, but still same issues. My theory is that the transmission (which is just under 2 years old) got screwed up because I was driving it w/ virtually no fluid. He's saying I just need a new sensor. I'm bringing it back AGAIN today after work. I feel like I'm being taken for a ride, but I can't do anything about it.