Well, the day I've simultaneously dreaded and anticipated has arrived (well, technically... I still have to go to bed and sleep before the big project in the afternoon). I'm stressing out badly, if only because this is one of the few tasks I can't do alone (at least, not without risking damage to the new tub), and tomorrow's the only day my friend can come over to help me until... well... about 2 weeks from now. So it has to be done tomorrow.
The biggest problem I'm seeing right now is the apparent fact that the old bathtub's P-trap is about 3 inches too far back. If you look at picture 1, you can see how I calculated it:
tub edge to back of overflow drain: ~5.75"
overflow drain, front to back: 2.25"
back of overflow drain to tub edge: 3.5"
subtracting .75" (half the 1.5" pipe") from 5.75" suggests that the new overflow drain's pipe is going to be approximately 5" back from the stud. The old pipe was apparently ~2.25" back from the stud.
Now, for the fun. As far as I can tell, I've removed everything from the old drain that can be nondestructively removed. It looks like everything from this point is going to involve cutting and gluing PVC.
The fun part, of course, is the fact that the two vertical pipe sections are both inside the ~8" diameter hole formed in the second floor's slab (the bottom of the hole is the top of the living room's drywall), so getting a saw in there won't be fun, and would probably require buying some expensive new tool(s). HOWEVER, replacing the entire P-trap won't be much more fun, because it only has about an inch before it disappears into the slab.
I should mention at this point that the bathroom is actually a slab cast on top of another slab. The builder formed the second floor's slab, then poured a second slab (delineated by the wood around the drain pit, and elsewhere around the bathroom's perimeter) with the drain pipes embedded inside. In a pinch, I could probably pry away the board the drainpipe passes through to get another half inch of pipe to work with, but for obvious reasons I'm nervous about cutting the pipe, because if I cut too much, the only way more pipe can be exposed is by breaking away concrete (at god knows what cost). The new 1x2 boards you see at the very top are coming off... I only needed THEM to dam off the area around the self-leveling concrete I poured yesterday.
So... any suggestions for which pipe to cut tomorrow after I've confirmed that the trap needs to be shifted 2-3 inches further from the wall? Is there any easy way to cut a "pipe in a pit"? Or is my only real option to cut away the entire trap, preserving as much of the pipe sticking out of the slab as possible? What's the absolute minimum length I can have before I'll have to pry away the old board used to form the second slab to expose enough for a proper glued joint?
By the same token, is there any good way to do the new trap in a way that will leave the door open to a third new drain, put in by me or whomever owns the house 25 years from now, without requiring slab breakage? Perhaps a PVC-to-[brass|galvanized steel|something-non-glued] fitting, so that going forward any new trap can be screwed on or compression-fitted instead of glued? That's my biggest objection to gluing a new trap into place... once the new trap is glued, there won't be enough exposed drainpipe left to ever cut THAT trap off and glue a new one to replace it.
Alternatively, are there any good ways to "cheat" and somehow use another section of pipe between the old trap and new drainpipe's bottom to shift it ~3"? God forbid, maybe a flexible pipe kind of like the ones they sell to make connecting sink drains easier?