If I'm understanding your post correctly...it depends on the construction/design of the pan. If the pan can't bear the weight of the wall framed right on top of it, probably the easiest would be to build a 2x6 wall and notch out the bottom of one side of the studs to go over the curb. Example...let's say your curb is 4" wide and 3" high. Think of the stud standing vertically. You'd want the notch to be tall enough to clear the curb height, say 3-1/8" to 3-1/4". You'd want the notch to be deep enough to cover most of the curb, but not so deep that the stud is too fragile after it is notched. So for this example make the notch 2-3/4" deep, leaving 2-3/4" of the 5-1/2" width of the 2x6 uncut. A 2-3/4" deep notch will cover 2-3/4" of the 4" of curb width, leaving 1-1/4" of the curb exposed. In the drawing below, think of the "x" as the stud and the "o" as the pan. You can see how the stud is notched out to accept the pan. The "z" is the sole plate for the wall. You'll have two sole plates. One at the bottom of the wall that will sit on the bathroom floor, one at the bottom of the notch that will sit just above the top of the pan's curb. "c" is the cement board and "t" is the tile. xxxxxxct xxxxxxct xxxxxxct xxxxxxct xxxzzzct xxxoooooo xxxoooooo xxxooooooooooooooooooooooo zzzooooooooooooooooooooooo When you add the cement board and tile to the inside face of the wall, it'll build it out a bit, but a bit of the edge of the curb will remain visible. How much depends on the style of the curb, if the edge is closer to square, or if it has a generous radius, etc. If you use 6-mil poly in that wall, seal the bottom edge of it to the top of the curb. In this case, your curb is 4" thick, the notch is 2-3/4" deep, leaving 1-1/4" of curb exposed. The 1/2" of cement board, the 1/8" thickness of thinset, and the 1/4" thickness of the tile will cover another 7/8ths-inch of the curb, leaving 3/8" of the curb exposed. One other comment...if the inside edge of the curb is dead straight then all will look fine. If the inside edge of the curb waivers in and out a bit, that 3/8" of curb might go from 3/8", to 1/2", to 1/4", etc, as the curb waffles around. The more your curb waffles in and out, the more curb you need to leave exposed to compensate for the waffling. With 2x6 framing, this will be a "thick" wall. A great opportunity to use the stud bays for storage. A shower niche on the inside, or a tall "medicine cabinet" or "storage pantry" built in to the stud bays on the opposite side.

In hindsight, a wider niche would have looked better or centering the smaller niche in the larger tile, but it really isn't worth changing. Next time you design/build a shower you won't make that mistake again, right? Probably not, but you will probably make some other mistake!

Typically we match the bathroom floor to the shower floor. If the bathroom floor tile is too large and slippery, we'd cut it into smaller pieces. Do make sure that your shower floor tile is not slippery when wet. There are products that you can put on the tile afterward to add some grit if needed. Your shower most likely has a threshold and make sure that is well-delineated so you won't stub your toes. Keep in mind that natural stone requires more maintenance and is not as resilient as porcelain tile.

"What could cause this to happen?" It's more an issue with the hinge screws not being anchored to the wall framing versus the marble not being properly installed. With the weight and moment of the door pulling on the upper hinge, and with the hinges only being secured to the "marble slab", it caused the slab to separate from the wall. " How can it be fixed?" As has been discussed,if you can add blocking from behind, do it. Reset the slab so it's tight to the backer, then run longer hinge screws through the slab and into the framing. " Who should fix it?" Your GC from the original build is responsible for the details, or in this case, the lack thereof. He should have seen this coming. Ultimately, the GC is responsible for the failure. The guy who installed the marble, it's really not his worry. The glass installer, he should have known that screws set only in a wall slab would not be sufficient long term. The failure is on the GC and the glass guy, partially on the glass guy for not raising an objection. The repair? You'll want to hire someone new to install the blocking and repair the drywall. If your GC is still around, ring him up. Then have the glass guy back to properly anchor the hinges in to the new blocking.