Any plumbers in here?

I think I am gearing up for a multi-thousand dollar repair job in my basement bathroom (enclosed glass walled/plastic based shower stall on a wood platform tiled over, standard toilet, standard sink), but I guess I am hoping for some wisdom that might ease the pain.

Further details of the bathroom:its in the SW corner of the basement, so two of its walls are outside walls, one wall has an outside facet, and there might be some of the "plex" tubing running inside the wall behind the shower stall, from the upstairs kitchen (though I doubt this plex is in any way part of the issue). The shower stall is against the south wall, but shares a wall with the toy room (shower is in SE corner of bathroom).

History of this bathroom:

I think it was put in after the house was built, can't be certain but I think I have detected certain little details to lead me this direction.

For the last couple of years, the shower has had a bit of a problem with a leak, as first detected by the carpet in the toy room getting damp at the edge.

So I quaranteened the shower, waiting for when I could fix it. The carpet easily dried up (never gets wet unless the shower is being consistently used). I ripped out the first shower stall, reworked the pressured plumbing, re-installed the first shower stall, special focus on making the coupling to the drain work well. Carpet started getting damp again. So, I ripped out the first shower stall and replaced with the second shower stall. Calked it all incredibly well, wife snuck a plumber in while I wasn't looking and he did the same. This process did stop the drywall getting damp at the corners of the shower (this was an issue I didn't mention earlier, but its completely resolved so no need to detail it further).

But, the carpet still gets damp with shower use. So, I popped the baseboard in the toyroom, cut out some drywall to examine the backside of the shower stall and under the platform of the showerfloor. It seems that the concrete is damp, but only the bottom 1/16 inch of wood touching the concrete shows any signs of ever getting wet, everthing else is dry. Further, the plastic cuff (this is the drain that constitutes the beginning of the house's plumbing, that I couple the shower floor drain too) seems dry (but I can't reach it to feel it). I run the shower for 10 minutes and see no obvious changes in moisture.

Its at this time that I turn my attention to the fact that the floor tiles in the bathroom (mortared to the concrete, not the wood shower platform) seem to be wet underneath as well. You get a squishy sound if you step on one of them in particular, the grout is always wet looking, you can even get water to bubble up through the grout if you step on the squishy tile. Now, this tile is near the SW corner of the bathroom (remember: the two outside walls). But, its not exactly in the corner. It is at least one tile away from either wall. While still showing signs of some dampness, the tiles adjacent to the wall are not the wettest tiles. The wettest tile is also not directly next to the toilet (but close), the sink is nowhere near any of this.

Finally, we had a big storm yesterday. My wife thinks its because of water from the outside leaking in. No cracks in the foundation I can see from outside, ground slopes away from house there. Further, if it were water from the outside I would expect the wettest tile to be the one next to the wall, and it also would not explain the damp carpet at all (its not damp as no one has used the shower recently).

I think this wettest tile may mark where the shower ties into the toilet and that something is wrong with the coupling (collapsed or something else, no trees near the house). How this ties into the damp carpet is hard for me to figure out at this time.

So, after all of that....any ideas how I should proceed????

First, I have brought in a shop lamp and heavy duty fan so I can dry everything out as much as I can. Further I have made the whole bathroom quaranteen and turned off all pressure plumbing to the bathroom. I haven't drained the toilet though as that would make everything stink....but if its not flushed, the drain passed the trap should dry out right?

Then I will experiment with some long showers and see if/how the wetness starts to re-appear. Then I think I may need to shove a camera down the drain.

Then, I think I will probably be stuck having someone jackhammer up my floor and redo the plumbing....oh I don't want to do that myself (did it in the last house, and I don't have that sort of patience anymore).

Thanks in advance for any help you can offer.

John