[Ziggythecat]

Good news is, the repairs and rebuilding is covered by your homeowners insurance.
I believe it is one of the most frequent claims.
Assuming it’s a halfway decent policy, they won’t pay for the part that failed (the pan), but the rest of the rebuilding of the shower and surrounding damage should be covered.

Your plan to construct a temporary membrane sounds workable.
You could use gorilla tape to hold it to the walls.