Crumbly grout is usually from using unsanded grout in too wide of a joint. In general, unsanded should only be used in joints less than 1/8" wide. For joints 1/8" or wider, sanded grout should be used. I have seen grout fall out where it was mixed with too much water, and when placed it was not really worked into the grout joints. So the joints were not completely full, and when the excess water evaporated, it left behind a sponge-like grout matrix. It would simply chunk out. Changes in plane, like a wall to a tub, are usually caulked. That's because dissimilar materials can move independently. Rigid grout would crack. Flexible caulk can flex. To make the caulk less obtrusive, there are color- and texture-matching caulks out there. Example, you use sage green sanded grout from "X" company, they might also have a sage green sanded caulk to match that grout. It might not be a perfect match, but it's better than picking any caulk off the shelf. Gas seeping through grout and staining it? Ehhh...I'll pass on that. I've heard a lot of strange ideas while mediating the blame game in the past, but I don't ever recall that one being brought up. If it was a mud bed, moisture evaporation can carry dissolved minerals from the mud pan up through the grout, and when the water hits free air and evaporates, it can leave mineral stains or mineral grit behind. If there is a water leak at this window, your stains could be a variation of that. If it's an orange stain, it could indeed be rust. It could be a nail head used to fasten the cement backer board that is rusting, etc, and it's wicking through the grout. Or something else. Iron in the water, etc.

I know I've seen some sizable niches here in the past year or so. Do a search at the bottom of the page for niches and see what you can pull up. I'll post some pics of the niches in our three bathrooms. Our niche sizes/placements/orientations were based on various factors; shower storage needs, tile shape/size, behind the wall restrictions (pipes, wiring, studs....). Guest bathroom niche: Hall Bathroom niche: MB niche: HTH!

I don't do tile for a living but I do have just such a tub, which I tiled around four years ago. I did not use any sheet metal (don't see the purpose), but I did use tar paper behind the cement board. I think stuck it to the (minimal) tub flange with something... a bit of roofing cement, maybe? I can't remember anymore. I used Hardiebacker instead of Durock for the tile substrate. I chamfered the back bottom edges of the Hardiebacker so that the inside faces of the backer could be a bit closer to the tub deck (since the flange is too thick to be pinched between backer and studs). I filled the gap between Hardiebacker and tub deck with clear siliconized latex caulk and let that caulk dry completely before proceeding with the tile work. After setting and grouting the tile, the vertical corners and the joint between tile and tub were caulked with sanded, color-matched caulk from the grout manufacturer. After a few days' curing time, the grout was sealed with Miracle 511. Four years later, the installation still looks great. If you caulk the space between the Durok / tile and the tub itself, how does the moisture get back to the tub? Grout and Durock / Hardiebacker are both moisture-permeable. They are "waterproof" in the sense that water doesn't hurt them, but they are not waterproof in the way that a fish tank is waterproof.

Ask your structural engineer about the weight of a porcelain over cast iron tub like those from Kohler. Get a licensed, insured pro to do the installation. Check references and go see their tile work. An acrylic surround and tub is fine unless tile and cast iron tub are more typical for your neighborhood.