If memory serves, the products that the shower is made from exhibit what is
called 'cold flow'. i.e., when you tighten a not/bolt configuration around
this material the pressure will cause it to migrate out/away from the
pressure.

As such, the primary sealing in this joint needs to come from some sort of
bonding agent. I think the post above re silicone caulk is on the right
track.

It seems like there should be easy answers to this since there is nothing
unusual about your configuration. It seems like the culprit would have to
be the shower base being too flexible which does lead to the issue of
whether there are already small cracks that you haven't yet dicovered that
either (or both) resulted from too much flex or even contribute to too much
flex.

In my town, the Home Depot I go to has an ex-plumber in the plumbing
department that I can pose questions to. If you know of a plumbing supply
house, you can go ther and ask but the best time to go is about 9:30-10AM.
Don't go there when there are a lot of licensed plumbers around (like the
first part of the day when they are there getting materials for their trucks
or their first call). The staff will sometimes be a little heinky about
helping you much in front of them. OTOH, sometimes the people around will
overhear and toss out suggestions. I think some of it depends on how strong
unions are where you live.

I thought the idea of a hard polyurethane foam was an interesting idea (and
possibly the only 'effective' one for flex) but that tends to make any of
your solutions **extremely** difficult to revisit and given that you have
drywall underneath you'd probably have to be putting in some sort of
(removeable) cross-member between the two relevant joists that you'd need to
have not be flush with the drywall (to accommodate flex in this member and
avoid it popping your new ceiling drywall joints). blah-blah-blah...

G*d*mn I hate plastic things - especially in plumbing. With the exception
of PVC tubing/piping, it always fails. I have not even used PVC on supply
lines because I am suspicious of it. I am still using steel and copper for
that. I would feel comfortable with PVC supply lines only after seeing this
stuff in service for another 30 years or so. It is amazing that there is
nothing in plumbing (or many other disciplines) that some *ssw*pe won't try
to make out of plastic.

The problem is what people consider to be 'serviceable lifetime'.
'Contractors/remodelers/investors' only need 6 months to a year to get the
unit sold. Even homeowners don't need long because nobody seems to live
anywhere for longer than 5 years. My house is 85 years old and I am
planning/hoping to live here at least another 30 years. I want my repairs
to **last**.

What a PITA problem! Good luck. Let us know what eventually works.