1 Seems like a good idea on the poly sheet tucked into the drain lip, don't see how that can hurt.
2 You can use the studwall to hold the poly in place. You could use adhesive, too.
In my basement I recently had a similar scenario on a 10 ft ceiling. I had a 2x4 top and sill plate with 2x4 studs turned width ways in the middle so they're 1.5 deep and used the remaining 2" for 2" xps.
The xps is solid so no wood contact with the wall, no thermal bridging, no holes in the block wall, and saves 2". I could have also filled in between the remaining 1.5 inch cavity with xps to get like an r17 wall if i felt like cutting the pieces. I did run 2 outlets in this wall which i had to carve out of the foam a bit.