Plumbers....adding a shower drain questions

Not a plumber, but how are you gonna clean that trap out, once it gets clogged with hair and soap. IIRC the floor flange Is held to the floor from the bottom with a large diameter nut, and the whole thing cannot be disassembled from the top.
So, IMO, that takes precedence, cuz teenagers don't give a hoot about your drainage, and will plug it up within two years.
I think you might want to elevate the floor, and move the trap out from under the base,lol.
My wife plugs ours up about every 5 years. Lotsa slope is your friend, else the soap cakes up in the pipes too. And if you flush it further down the pipe, it will just cake something else up. I hooked my washer upstream from there, to flush the line, semi-continuously,lol. It seems to be working.... touch-wood. Lord that woman uses water.

Nice pics BTW, makes it easy to see what you're up against.