"Could the cracking of the grout be from moisture from the opening of the concrete floor under the shower? i" You pretty much hit it on the head there in that it is moisture related. For the most part, it could be from one of three things: 1) It could be moisture or moisture vapor coming though the unplugged hole, and wicking outwards to the curb. But if that was the case, in terms of there being significant moisture, you'd probably see signs of water presence or water damage at the base of the curb. 2) It could be the wood curb simply wicking moisture through the slab below. The thing it, as best as I can read the photos, it appears that the majority of the swelling is on the top of your curb. The tile on the outside curb facing could have the bottom edge locked in place by the floor tile, in that case it could be the floor tile that is preventing the bottom edge of the curb tile from blowing out. But to me it looks like the curb tile sits on top of the floor tile, so if the bottom of the curb tile wanted to blow out, it would be free to move. Correct? Incorrect? Based on that assumption? I'd guess that: 3) In the first two photos, it looks like there is a metal track or trim on top of the curb. If that track/trim has been fastened through into the top of the curb, it's possible that moisture is getting to the curb through the fastener holes. The top of the wood curb got wet, then expanded, and it's that expansion that is blowing the curb apart. I don't know how the membrane was detailed at the curb. It should be wrapped up the inside face, over the top of the curb, and stapled on the outside face of the curb. Any fasteners through the top of the curb could have penetrated the membrane and allow moisture to get to the wood. I've seen a lot of curbs where the membrane never completely made it over the top of the curb. In that case, it could be as simple as water intrusion through the grout lines on top of the curb. Again, that's based upon my assumption (cough, cough) that the floor tile outside the shower is not locking the curb tile in place and the bottom edge of the curb tile is free to move if it had to.

Do I understand correctly that to inspect/unplug weepholes, we will have to remove the stone around the drain? Is there any hope of doing that without destroying it? How do we unplug the weepholes? Are there any special tools required? It CAN be done, even as a DIY task. You need to cut the grout joints around the tiles you're going to remove, and then use a hammer and chisel. once you have the tiles out, you need to be very careful not to damage the pan membrane. All it takes is a pin prick, and the whole thing is coming out. Once you clear the weepholes, though, you need to tape them off and water test it before you put it all back together to make sure it's still watertight.

@dostk did you ever resolve this? We had a white and gray honed marble mosaic installed and after grouting, the whole area looks completely different. The stone doesn't look anything like the uninstalled mosaic, now it has almost a dull/frosted glass (not shiny) look to it ... is the best way I can try to describe it. Basically it doesn't even read as marble to me anymore. As far as I know, our contractor's installer did not seal it before hand (it was still in the boxes and packing sleeves) and used a sanded grout. The option of sanded or unsanded was never mentioned to us and as newbies we didn't know to ask. Is there anything that can be done to bring the original look back or is this just expected and we should have done a mock up as someone mentioned before?

Thanks for the comments. all I can say is when i put my nose up to the edges of the glass where there is silicone, I can smell the odor I am referencing. Same with the fixtures. Very strange that Thermasol who makes steam generators would recommend a silicone type that isn't approved for use in a steam shower? With all of the ineptitude I have experienced through this process, I suppose it is possible but this is what they told me they include with their steam control and steam head for sealing inside the steam shower. Heinkel is closed at the moment but I will call them tomorrow to ask about the Loctite SI 595. As for waiting 30+ days before steaming, the build out, tile and glass install were complete prior to April 15th. The only thing that is one month old is the seal around the fixtures inside the steam shower as the plumber originally used water-based acrylic seal, which I know isn't approved for steam showers so I called Thermasol to get a silicone rec, removed the acrylic myself and hired someone to appy the silicone around the fixtures; that part was done about a month ago. I am still smelling silicone along the edges of the glass and around fixtures. I did not smell a smell until after the glass was installed so I do not suspect it to be a thin set or mortar issue. I first used the shower in late May and the steam shower 2 weeks ago for the first time; the smell was too intense to continue. Does anyone have advice on how to air out cured silicone smell from a steam shower?