The problem with a Kerdi shower is that it's difficult to pop tile off the membrane after the thinset has cured without damaging the membrane, so resetting the curb to have proper pitch back to the shower isn't a simple repair. That said...it's still up to the GC to provide remediation. If you want a pitched curb, tell him so. It's between him and the tiler to make things right, and not at your expense. If you can settle for a dam on the curb, the only warning I'll give is that if water is pooling on the curb, it'll still be pooling there with a dam, so you might end up wiping it down with a towel after each shower. You could opt for a dam to see how it works out with the option of going with a rebuild of the curb if the dam doesn't suit you. And a common come back from the tiler is "The Schluter curb isn't pitched, the top is flat." So? Some guys shave the curb to have a pitch, some just add extra thinset under one side of the tile to pitch the tiles. Whatever agreement you come to, consider putting it in writing, especially since this had turned into a "he said, she said" blame game. And no worries on your low-tech design methods. I laid out the floor plans and basic elevations of this house with sidewalk chalk on a driveway.

If it appears to be coming out of the bottom, chances are good you're right-- it's a leak. If it was what I thought it might be, you'd see the water coming from the top of the liner a few inches up the wall. As for it being an odd shape, it sounds like you have what's called a "neo-angle" shower-- two straight sides against two walls in a corner, with two short walls, or a curb coming in at right angles to the back walls for a short distance, and then connecting diagonally to each other. If that's the case, you might look into something called "The Kerdi System". they have foam bases premade that you can use and waterproof yourself rather easily and then DIY the tile, as well. I've included a link to the product page. On it, you'll find a couple of streaming videos, and believe me when I tell you, it IS as easy as it looks to use it. Here is a link that might be useful: Schluter Kerdi

Update: Managed to get a plumber to come in to perform a second opinion. I wasn't able to be there again (I live 7 hours drive away) unfortunately, but on short notice, and a looming inspection contingency deadline I thought it was the best option. Plumber isolated different areas of the bathroom to see what would generate a leak. It is, in fact, the grout (or lack thereof). I suspect that the tile is placed directly over green board. The sellers have offered to give us a credit at closing for the repair work on the tiles ($350), but I am insisting that they do it themselves. I can imagine that this could turn into a much bigger/more expensive repair once tiles are removed and greenboard uncovered. So, I don't think their credit at closing is sufficient. We'll see if they go for it. Thanks for the help everyone!

I think the most simple attempt at a quick fix would be to make a dam at the inside edge of the tub from the wall with clear Bath and Kitchen silicon caulk. That should channel the water back into the tub before it gets to the outside of the tub. The more secure and longer lasting fix would be to install a sliding shower door.