It's not impossible that these issues are related: I don't *think* the sewer gas is directly causing the respritory issues because if it was strong enough to irritate it would have been too strong to tolerate. But that pipe would have increased the humidity level, and encouraged mold.

I think I'd start with just painting the walls a soft white or very pale taupe maybe. To me it's too much peachy beige going on. Is the wood trim consistent throughout the house? If so, I'd leave it (unless you want to tackle the whole house) and the vanity. If not, then you could consider painting the trim, vanity, door, etc. Assuming you don't want to re-do the flooring, I'd want to know if the tile runs under the vanity before considering replacing it with a single sink model. The mirror situation is tricky. I'd be tempted to just leave them or maybe just remove the one on the angled wall and have a single mirror on the flat wall, even though the second sink isn't centered on it. I think no matter what you do, if you keep the two sinks on this vanity, the mirror situation is going to be a little weird, but that's ok.

I'll be a contrarian. I would never knowingly rent a room with a shared bathroom in the hall. That would be a complete pass - I don't even like having to use a hallway bathroom in a sleeping situation in which I would share with friends and family because if I need to use it in the night I would have to wake up more/get more decently dressed and I hate having to shlep stuff to take a shower or perform normal grooming stuff in a bathroom. I don't have the horror of viewing a toilet from a bedroom that some houzzers seem to have. It certainly wouldn't bother me on a temporary basis. I have a master bath in my home and I can't remember the last time I actually shut the door. I don't have issues with smells or humidity but perhaps that is because my Toto washlet has a built in deodorizer AND I have an excellent Panasonic fan which runs based on ambient humidity levels. I don't generally even have condensation on shower doors or mirrors after exiting let along huge amounts of humidity wafting through my bedroom. The question really is one of economics. There is a cost to installing the bathroom and OP would need to decide whether they would make up the expense by having increased rent and/or renting it out more frequently. Certainly any prospective renter would see a picture and determine whether they wanted the ensuite with a curtain in the room or use of a bathroom in the hall. Also resale value is a red herring. At such point that OP wanted to sell it would be relatively inexpensive to convert the bathroom into a closet if having the ensuite in that room was a negative.

I like stacked to keep things ismple and also matching grout for the same reason I sometimes do an aceent strip not centered on the long shower wall I do it vertical and use that same accnet tile for the backsplash for the vanity to adda bit of bling. I do the same pattern for both floor snad shower walls .