OMG you're scaring me Cleveland. My shower stall was molding so I also tore it out thinking it would be soooo easy to rebuild. I was wrong! I've just finished demolition like you, but i haven't dismantled the top part of the drain pipe.
The old plumber pretty much built a cement slope on cement ground and I was hoping that I could use a kerdi adaptor to attach on to it. But my problem is that the original builder pretty much tarred the whole area and put a pvc liner on top of the cement slope. I dont know if I can remove the pvc liner cleanly enough out the tar cleanly to attach the kerdi adaptor using kerdi-fix (glue) and hope that water does not leak out..
I will post a pic of my shower stall later. The part about the water draining very slowly sucks... are you sure ur pipe isn't clogged? Also, u know the horizontal piping right before the trap? Well, your drain has to stand like, above that horizontal piping. It's gravity that pulls the water down the pipe, meaning that there will always be water in your trap, because it sits below the horizontal piping.
Good luck, let me know on your progress. I will probably start a thread of my own too.
I'm definitely a plumbing novice, not a plumbing expert. I'm not sure but don't basement drain lines require a 1/4" downward slope per foot, the same as anyother horizontal run of drain? See attached crude drawing. Maybe as donutboy mentioned, you have the drain horizontal or maybe even slightly upward sloping, making it drain very slowly at best. Good luck, and surely get more opinions on this!