1) reverse is always hard to do as non synchonzed..at least I need 2 ..3 attemps..and all is brand new
2) check the setting of the shifter according spec (again)
3) moving a Panter up and down a trailer is hard work, so "slipping clutch car movement" applies there at least.
4) if no milage..you are for high percentage good....
5) check fluid level in main clutch container at front
6) drain SLAVE in engine bay again
7) check clutch play ..as alread suggested. There is little good from bad play on this car clutch
8) fileing a claim is a least something for the books and you should do. In my case they did not pay as any engine and drive train issue where not covered..always surprise in life..
..crossing my fingers!