Sophie, what would you do if you were me? it's a 56" by 108" shower (counting the bench seat). Tear it all out and start over?

Any other thoughts?

Yes. Gut it and start over. From someone who has actually been to the Schluter training.

How did you find yourself here in this situation? The error is in the hiring process and id suggest you put the brakes on & try to salvage the $200 drain and ask advice here about HOW to vet and hire a tile professional and what type professionals it may require to build a barrier free linear drain shower. PROPERLY and its starts with hiring properly

Okay folks, how about being more specific rather than just saying "Clueless, etc."
I think the OP intuits that there's a problem. S/He mentioned the slope... so what specifically is the issue so that the OP has something more solid to go on.

You don’t need to rip out the walls or likely the bench. Just the floor. A foam shower base is likely the most likely least expensive option to get a barrier free shower.

People who were arguing that homeowners shouldn’t need to be educated about construction details are now saying that you need to be educated enough to tell who’s going to build it right. Hmmm. Unless we are once again defaulting to the position that paying a bunch of money (without knowing what OP paid) ensures excellent results.

Rip out the walls also. The wall on the left CBU end is floating in between studs not to mention the 2" gap at the inside corner and the right wall has a gap for some unknown reason. Rip out will be easy as there are no where near enough fasteners. (8" spacing required) All prep work is to be completed under the Kerdi membrane per Schluter instructions. The drain does not appear to be a Kerdi drain as the membrane would be attached to the drain flange. Seeing you are on a cement slab a foam base is not likely to be helpful. I would have to assume the cement was removed in the shower area to lower base for a barrier free installation. Then again with this installer who knows. I would not allow this contractor a second chance. Clueless is the correct term. Sorry.

It looks like he left the existing concrete in place and is trying to slope the shower with that sloppy mud work? It doesn't look like he got too far where tearing everything out and starting over would be too costly (besides the labor or down payment of course)

For a curb-less entry to work with a concrete floor, you either have to raise the outside floor area in order to be able to slope to drain (which is not usually a practical option) or remove the concrete in shower area and slope a mud bed to drain.

I would recommend trying to get the channel drain in front of bench so that the tile work is easier to install (could use large format tile that way too)

Sorry you've got such a poor installation done...but there is no way this guy would be able to complete this project without major problems down the road (maybe in the first shower!)