Wow! Was there any access from below? Either in a basement/crawlspace or through the 1st floor ceiling? I had a similar leak to repair once, but I was able to do all the work through the shower valve hole and from below. I cut the copper pipe out, and snaked some PEX up to the valve.
You know, you could probably temp this by cutting out the FG right up to the fake tile lines, back boarding the missing drywall and replacing it, and installing ceramic tile bedded in urethane or silicone sealant till it squeezes out the seams then cleaning up with alcohol. It would probably work for a while, at least you could take a shower again. I think I would rip out this POS. Put in a shower pan and screw up durock or wonderboard, and install ceramic tile were it mine. Tiling is really not all that hard and it looks a lot richer than a cheap ass FG insert.
" Most people would rather die than think, and most do " Bertrand Russell
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