re: P trap help - how would you repair?

I honestly think he just needs thread tape. I can’t tell exactly what sort of pipe and threads he has, but he said it screws all the way and still leaks. Which likely will be solved with thread tape or pipe dope. Keep the rest as is.

He is basically trying to make a water tight seal without tape, glue, dope, or anything and some threads that likely were as you said from different kits and not designed to screw together.