Plumbing – Changing basement tub drain to shower drain – double 90 degree while horizonal, is this bad

A shower drain needs a trap, so check under the cap on the left to see if there's a trap or a straight drain. If it's straight, then your assumptions are likely right that it's a toilet and the other one is a clean-out (fix that cap, you don't want sewer gases getting it). If you don't have a shower drain here, then I'm guessing it's only roughed in for a half bath.

I don't imagine the 2" would be anything other than a vanity drain.

With the "vent," I'm left wondering if it's really a drain since I see a clean-out installed on it. I'd recommend tracing that line up the walls, as best you can, to verify it really is a vent.

As for where the horizontal line is going to the left, perhaps a washer dryer, or perhaps the plumber screwed up the T and it should be pointing to the right for the sink. It's hard to tell from here.

That pipe that is pictured above the floor may be the beginning of a vent pipe. Yes it does look like there is a trap below the floor, from what I can see, a little more dirt removed would have been good, but it looks like it is there. If the tubs bottom is high enough, you will not need to remove more concrete, the drain of the tub can set above the concrete floor, most tubs do not allow that. Most likely you will need to break out about 8-12" more under the tub to set the drain. Typically the trap is glued in last so the pipe from the overflow and tub drain is a straight shot down into the J bend of the trap. With the way it is assembled now, you will need to add a few otherwise unnecessary elbows to get it to connect. If you leave it that way add a clean out real close in case it clogs. More elbows equals easier clogs.