I think that the black slate 2 inch mosaic tiles used in the base of the shower was just the wrong material to use.
Check the caulking at the change of plane joints at the base of the shower.
I would re-caulk with a urethane or silicone black or grey caulk to prevent water getting into the pan.
Most sealers and topical will probably fail in this situation
You may want to try a product like stone glamour or sigma seal made by sparks,they may fail as well.
If it were may shower I would replace the base with ceramic faux slate-but larger tiles.