Transmission: Want to sue my mechanic... but I'm I right?

You paid to have a used transmission of unknown quality to be installed. You got a used transmission of unknown quality installed. Now you know the quality of the transmission. Live and learn, move on. I really do not think you have much legal ground to stand on. Just my opinion.

I think your decisions had a lot to do with the quality of the repair. 1. It sounds like you wanted done as cheaply as possible 2. You selected a shop that can’t seem to provide a written bill (how did you select this shop?) 3. You scheduled it when you could not be around for a month.
Consider using this as a life lesson and learn from it. Although the shop may have some fault, admit you had your share of fault, too. Just go from here to fix the current problems. And be careful about using the same shop.

Quote; The leaks come from the yoke seal and the seal that hooks up the driveshaft and the transmission.

This sounds like a rear wheel drive car and it would in no way take 13.5 hours to replace those two seals. I think that there is muck more to the story that we are not aware of.
I could do that job in 2 hours!!!

But, I feel from the statements of the OP that he went back to the mechanic demanding more than what he was entitled. Not just seals but a new transmission maybe. That could have brought the price and labor up to the 13.5 hours for the mechanic to recover his losses.

The mechanic may have been willing to install new seals for free, but the OP insisted on a new tranny as well.

The Op should have noticed the leaks…even on gravel…much sooner. I understand he was out of the country, but that left over two months to notice the leaks.

I also wonder why the Op would record his conversation with the mechanic. Way he looking for evidence, just in case the mechanic did not refund his entire bill paid!!!

Makes you wonder when people are ready to jump into a law suit at the drop of a hat.