Trying to fix that pan from under the floor would be like trying to fix a leaking roof from the attic.
Still going to leak, still going to keep on rotting out whatever is under the pan, molds going to set in. But at least it stopped dripping.
The flex seal looks like really cool product.
I found aqua epoxy in HD and put them over the cracks. So far I have had water in the shower pan for a few days and it seems working. I guess I will know if it really works in a few months :-)