How do I go about proving the mechanic ruined my transmission

I got my car in Dec. 2019 Before I bought it I had everything checked out . The car had one owner , it was kept in the garage , all regular maintainance done in the shop . 167,xxx miles on it and in immaculate condition . Leather seats, sunroof, nice rims, only a few scuffs on the paint. Engine strong, tranny strong, car never over heated or ran hot . A few months I started hearing a noise sounding like a belt . I do all my own machineics by the way and I am a good looking female. Lol! I could pin the noise so I took it to a shop we put it on a rack and proceeded to take off the belts and pulleys determining it was not any of the belts external. came to the conclusion that it was possibly the water pump well everybody knows when you change the water pump if you change the timing belt and so on. I checked around looking for a reasonable price to mechanic and in the meantime bought a water pump timing belt kit I found a mechanic/friend that has a shop and said he could do it the very next day I took it in at 8 in the morning and picked it up at 5. I was so happy that they had finished the job because my daughter was in the hospital 100 miles away I went home packed a bag and proceeded to my daughter about 30 miles out the transmission kicked like someone had hit me with a truck from behind. I had to be towed the remaining 70 miles then towed again to get home. Come to find out the mechanic had switched my transmission solenoids which are external and on top of my transmission when I told him about it he denied it. I never did receive any old parts back from the job he was supposed to have done which was all the way on the other side of the of the engine nor did I get a receipt what can I do it's been 6 months

Why do you think he swapped the solenoids? Why would he do that?

So this happened six months ago and no resolution from the mechanic? So how did you verify he switched the solenoids? It's his word against yours at this point. A few points: Most shops I frequent, I have a repore with the mechanics and not find the cheapest garage. Also most shops won't give you the old parts unless you ask. Most shops won't install customer parts as they won't warranty the work if it isn't their parts from their distributor. No receipt? That is on you when you picked up the car. I would go back to the beginning of your question and say maybe the noise was the transmission and not a belt. At 167K miles on a 21 year old vehicle, the transmission can fail at anytime. What should you do? I would have the transmission rebuilt and move on.