Hello all! I would like some advice on dealing with the following situtation.

We moved into our 1952 house three years ago. There are two upstairs bathroom, the master bath and the main bath; the master bath is directly over the powder room that's off the family room. At that time the faucets barely worked on the master bath shower, so we used the main bath instead. About a year ago, we had a plumber come in and replace all the fixtures (regular showerhead, two body sprays that were already there, and he added a hand-held shower).

We rarely use the powder room, so it may be weeks at a time before one of us goes in there. A few months ago we noticed that the paint and the skim coat of plaster underneath were starting to swell and blister. Because the main bath has always leaked into the kitchen cabinets directly behind the powder room, we assumed it was damage from that and told our boys to start using the master bath shower instead. We are in the midst of planning a kitchen renovation and figured that we would take care of the main bath problem when the ceiling was open for that.

Then a couple of weeks ago, we went into the powder room, which has now deteriorated markedly: almost all the paint and skim coat is peeling off and there is visible water damage and mold directly underneath where the master bath shower is. We stopped using it immediately. Since we're in the process of getting bids for the kitchen reno, we're showing the bathrooms to the bidding contractors; once we hire one for the kitchen, we will have them redo that bathroom and repair whatever's causing it right away.

What they've told me is that there are two possible causes of the damage. Either the shower pan is cracked and leaking (it appears to have been replaced at some point as the tile is different around the bottom of the shower) OR the damage is coming from something leaking in the wall where the fixtures are. While we never used that shower for bathing, I would use it occasionally to run water for mopping or to clean other things and never noticed a problem before the fixtures were replaced.

Should I call the plumber who replaced the fixtures and have him look? One of the contractors said he could take off the main plate and look inside with a mirror to see if something is leaking there. He brought in other guys to knock out the existing tile and replace it afterwards, so if he brings in someone else to knock out the ceiling below and see what's going on, should I be expected to pay for that IF it turns out that he caused this? If I have the contractor open up the ceiling and it's the plumber's fault, should I call him to come verify that and put the contractor on hold till he can come over and assess? It was hard to get him to show up on time for a paying job so I doubt he'll rush right over for this. If it's the shower pan, too bad so sad for me...but if it does turn out that he caused this, I'd rather give him a chance to make good on it somehow before taking him to court...but if this is his fault, I'm damned if I'll pay for renovating a bathroom I had no intention of touching but am now forced to deal with.

If you have any advice on the best way to proceed here, I'd love to hear it!