I'd just make sure the guys doing the tile install have used Kerdi before. It's certainly not nuclear physics, but I'd be happier if they had done one or two before you. If they haven't done one before, I'd want to make sure they sat and watched the Kerdi video completely...probably a couple of times. I'd also advise you to watch it yourself. You don't need any expertise to watch the video and double check your workers work. Again, I'd advise this if they've never done it before. If they've done a couple, leave them alone. Personally, I'd still watch the video and check up on their work. It would just be long after they've gone. You want a good install, but you don't want to be a pain in the @ss either. Laying the tile over the Kerdi can be tricky. Layering the different seams can make it tough to get a flat plane with the wall tile. Again, an expert should have no problem. My first experience...I'm not a pro by any stretch, but I do know which end of a hammer to swing...definitely had a learning curve.

If you do use a standard flat piece curb, you set the pitch when installing them. It's one of the few times when it's okay to install something "crooked". ; ) A fabrication shop can make them however you specify, but you'll pay extra. Sometimes you can find marble saddles or curb pieces that do have an extended bevel on each side. Depends on what the local places carry as "standard". Setting glass on a pitched curb top is not a problem. If you do plan on using ready made marble pieces, look of one with an extended bevel. A standard bevel may only be a 1/4" or 3/8" 45-degree chamfer on the edge of the stone. A "hollywood bevel" is an extended bevel, so it's a longer area of slope. Then there are "double hollywood bevels", where each long edge has a wide bevel. A picture may be worth more words, lemme see what I can find...here's a hollywood bevel Hollywood bevel: While most thresholds are 3/4" thick "door" thresholds, you could use those, or you can get the same material and style in thicker material if you want a thicker curb.

Generally, I recommend sealing marble. A sealer doesn't "waterproof" the material and trap moisture. A common misconception. A sealer simply helps to repel dirt & stains...making it easier to clean. You might try sealing a sample piece to see the result.....

That depends on the type of marble... I can spend $10 for a piece of Carrera marble with a bevel on each top end from the local Home Cheapo, $25 for Carrera with much nicer looking bullnose sides from my granite guy. I've spent well over $100 for a pure white Thassos marble saddle for a 5' long shower curb. Exactly. It cost me even more than that once for blue pearl. (175.00) If I can use a standard carrara threshold, I won't even charge for it, because what I lose in the material cost, I make up for in time savings. Anything more than that, though, whatever it costs me gets passed on to the customer, as it should be.