Thanks so much for your input everyone! Bevangel, special thanks to you for modifying my picture. It definitely helps to have a visual! I think moving the doors further into the hall will work, and should be very easy for the builder to do. We'd lose a little more exterior hallway space (the bedroom doors were originally at the dotted line), but having the longer hallway within the master bedroom will be more dramatic. I love this website! I'm glad I finally decided to join and stop lurking. ;)

Best case scenario exactly what airqual says - it's a sealant/caulking issue - you can recaulk the entire area and deal with the mold. If you have to tear out the tile work it gets more expensive quite a bit. I don't know what to advise with plaster walls - don't have those around here but I still think you probably have no choice but open up the wall to let it dry and get mold out. One thing I might try - if you can get the baseboards off the wall you could cut a hole in the wall at the lower level and see what is in the wall cavity perhaps this way (depending how high your trim goes, remember there is a baseplate). You could then patch with drywall as it's going to be hidden. but if you're not the DIY type at all, probably best to just call someone in. You have to pull the carpet out no matter what and padding and get everything away from that wall. If you do that and can identify which area looks "wettest" then that would give a clue where to start looking and I'd do that before anything else. Give servpro or other type business a call to come out and see the issue. They'll give you an estimate, I have no idea what they charge but they have all kinds of mold abatement sprays that will kill the stuff without having to totally destroy the wall. Sorry can't give you more help - I'm sure it's incredibly disappointing and stressful.

How about... Widen shower to 36-40" wide, filling floor up is no problem... Toilet to rotated so backed up to wall between shower and this new WC, and this slides into the current mirrored vanity space...this is walled into a WC with a (louvred) pocket door that slides toward the shower, allowing the light/fan switch to be on the wall to the right when you slide the door open... The space beyond the WC, into the corner above the current sink is a linen/storage/vanity supply closet...or a small seated level vanity with storage and a mirror for the young lady... 180 degrees from this is the vanity with sink, taking up the upper half of the mirrored slider closet... The remainder of the mirrored slider closet becomes a hanging closet or storage closet accessed from the bedroom directly... The main closet gains about a foot into the current walkway and moves the doorway onto the bedroom wall, mirroring the other door across the entry to the bathroom (into the remaining part of the old mirrored slider). The only real plumbing change is the toilet moving, and the sink moving is as easy as taking the current back into the wall (we hope) and around inside the new wall to rotate the position of the sink.

All the plans have a trade-off in room shape, cost etc. I agree that putting the bathroom in bedroom #4, the room width is the trade off. The bedroom width is currently 10'10". If I look at the adjacent master bathroom toilet cubicle, it's not much wider than the closet in bedroom #4. So the closet is probably 24" and the master bathroom cubicle 30". If the OP is OK with a 30" width then the middle plan may be the best one, since it only takes 6" off the width of the bedroom making it 13'10" x 10'4", and eliminating any door swing into the room.