I'm trying to picture your layout. Let's skip the bench for now. I've seen & ripped out a bunch of showers with a pre formed base and have always found it's not possible to waterproof the doorway curb. So, where the base ends and the threshold its self when the base is wider than the opening. (you probably didn't follow that, I barely did myself).
Can you post a picture or sketch of the area?
Jaz
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