some observations/questions from a "scientist"....
0. the suggestion to get a "real" plumber is valid, because your pictures and descriptions generate more questions than the pictures and posts can efficiently answer...a pro, on-sight would immediately see the most efficient solution...that being said....
1. if the pipe that is leaking goes "up" - and i think i see a banded coupler up there, then perhaps it is a vent with a bit of negative pitch? water is pooling in the pipe, but not blocking the operation of the vent.....? don't see the trap for the tub, or the drain run.....
2. it is so tight in there, that the elbo couldn't be "turned" to get a good seal, or it was wet to begin with? thus the leaking joint. see 'why now?' below
3. what is the water source? condensation on the inside of the pipe? slow draining tub which forced the water up the vent? if it is a constant drip, then really what is the source other than condensation. (fill the tub, then let it drain and see if it gets worse???)
4. it definately could be fixed with a full replacement..$10k in plumbing work is going to be matched by $10k in refinishing work - seems an inefficient solution. the simplest solution is the best (and by solution, i mean something that fixes the problem, not defers, or masks it.)
5. why did this start after the initial repair? did the foam move the pipe? perhaps the coupler just needs to be tightened? realigned/replaced?
what to do to fix the problem.... and lets call it a gambit to avoid the major plumbing work - given that a possible fix is so inexpensive, that it justifies the attempt.
since you are willing to try...i suggest replacing the observed problem joint,
but be aware that there may be an underlying cause.
get a long bend street 90, a 2' section of 1.5" pvc, and another banded coupler (plumbing supply, not homeowner store). Cut the leaking 90 out, right at the hub - hopefully the piece that is left will clean up with pvc primer (wear goggles) - let it dry. recreate the vertical piece and use a banded coupler to connect the old horizontal pvc to the hubless side of the street 90 (pros: is this legal, or does it need a nipple/tail?)
if the pipe is pitched the wrong way, and it can't be 'convinced' to pitch towards the drain, that is a problem.
pretty sure you could get a hack saw up there, or at least a blade in a quick handle.
total cost - small pvc primer, cement, a street 90, piece of 1.5" pvc, and a coupler...
as suggested, hacking around in there could cause more problems, so plan B must be in place if all goes wrong - ie, the pvc pipe just falls apart. so have some plugs to close up the pipes (you can always return them if not used). be ready to do some tear-out in the bathroom. perhaps the wall adjacent to the w.c. and maybe even the floor under the w.c. which would make replacing the bend easier....
definitely test the seal on the w.c.
do you have a date on the bathroom remodel? just curious. I love the half sawn log joists.
good luck.....