Plumbing – Decipher the bathroom rough in

You didn't mention if the drain pipe is accessible from under the floor. Assuming it does not pass directly through a concrete slab, the only safe way to terminate and seal the pipe from backwash and septic gas is to cut it off and glue the proper sized cap on with primer and PVC cement. If and when you need to reinstall the drain, simply cut off the pipe again below the cap and use a coupling to continue the pipe run back to a new closet flange. Do not use a screw cap. They are for temp use or pressure testing only. The hardware can rust, loosen and leak.

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.