- Just cut out the T and then drain/vent there.
- You don't. It should have been somewhere else in the basement. Not sure what they were thinking putting it next to a shower. You would put a backwater valve in a laundry or storage room.
- Who knows but it sure doesn't look like it's vented.
I cannot think of a logical design for your bathroom. The plumber who did your rough-in must have been high. You have a sink on the left then toilet... then in the corner is what? I guess you could put shelving there but I never design something to hold items to get pee spray. Also if the shower runs with wall it might stick out further than toilet. There is no good solution there.
My advice is to jackhammer up the backwater valve and move that into what will be unfinished space. While you have that ground broken up by shower, make sure it vents. Design your bathroom so that door is on the far left. You walk in and have the sink to your immediate left (might actually be able to keep the T in place). Then toilet after sink. Across from toilet is the shower - you are moving the valve so shower will be installed perpendicular to wall. To vent your shower go straight to the wall and then follow the wall over to sink vent.
If your backwater valve comes in on that wall then basically your costs are concrete and extra pipe. If it comes in somewhere else you might be able to just cut out pipe and reinstall head. You do not want this anywhere near a newly renovated bathroom.