Shower drain location

This type of shower should not have bounce to it even it has foam underneath to give it a softer touch. The showers are more often supposed to be installed with a heavy layer of thinset underneath to stabilize the pan and to build up any voids. Some showers are a little softer than others but they don't bounce or flex... or they crack and break.

At this point you are asking us internetters what you should do. You are trying to repair it, you are thinking about products and so on. Well you can fix the crack with repair kits. However you cannot fix the flex that is causing the crack. So your repair kit will work until it dries out fully looses its flexibility and then you stand on the area again and it breaks. Maybe you notice a tiny rip... maybe you don't.

Use your resources better - your time and money. The base has to go - for sure. Not only will it be a lot less time to put a new base in but the big issue is the potential damage you are doing to the rest of your house. Water can slowly screw crap up and you don't want a small leak in places you can't see.

To answer your question - you only need enough slope for water to drain. I suggest that you give up on the pan repair and post another question around how you can get that pan out or alternatives for your situation.

You have a blockage further down in the pipe that services your shower, toilet and sink. That's creating a sea of liquid that comes out at the lowest available point (i.e. your shower). Drain cleaner will be worse than useless; while a pool of poop may be nasty, a pool of caustic poop would be even worse. A 5' or so drain snake won't do it; you probably need something much longer, and a way to get it into the pipe (past the traps on whichever device you enter through). And, until you've cleared it, stop adding liquid to the situation.