No easy way as we can't even tell what it IS??

It's the corner of what, in what room? A bath? A shower knee wall?

Yes, it’s the corner of a pony wall in the shower.

Well the first issue seems to be the pony drywall isn't even plumb? Why am I seeing a stud?

I'd get that ripped back, and USE tile on the front edge of the pony. Then schluter at the corners. So yes, a do over is in order. Get someone different to do it. You may need a new top to the pony- what is the curb top in the shower? It should not be tile........you do not want grout on a curb top.

I’ve been checking on waterproofing and I believe it’s been done correctly. They had one tile guy that did all the water proofing and he had great attention to detail. He did our backsplash in the kitchen and it looks perfect. Unfortunately, he quit in the middle of the job and the guy that finished it is not detail-oriented…obviously.

I wonder if you could take off the top and side of pony wall and attach solid surface material instead to do away with the schluter altogether. You could then just silicone the corners.

IMO a rip out and a start over but please post a pic of the whole space . This satrted with the drywall not even being installed perfectly and IMO there should be the tile all the way around that pony wall but I need to see the whole space . So post those pics here just like you did already .I am not even sure that is schluter .

The plan was tile all around the pony wall, but we didn’t like the way it matched up the counter, so we asked to have drywall on the outside. They said it wouldn’t be a problem.
They’re working right now in the bathroom so I can’t get a photo at the moment, but here are past photos.

It seems like the current solution is just as mojavemaria suggested: take off the top tile and schluter and replace with solid surface. The wall is plumb, I checked it. I think it’s just messed up at the top where they added too much drywall mud. They’re going to scrape that down and hopefully we’ll be square again.

Having tile work undone is not something that should be undertaken lightly....

Oh, believe me, I understand. They’ve had to take quite a bit of tile off because of other errors. Fix Wedi board, reseal, etc. This section is the farthest from the shower head and I think can be removed the most easily, but I’m sure it will be a mess. I just can’t leave that corner the way it was. So sloppy.

How did they waterproof that shower and why is there a large window inside the shower and how was the window waterproofed?

So, I see the ponywall was tiled with the edge of it already buckled like that. You say midway through you all decided to re-drywall instead of tile that end. IS this the new drywall or the old?

That is drywall they put up. The pony wall was originally in a different location so they constructed this one.

We made the decision to drywall the outside of the pony wall before any drywall or tile was put up.