I am actually putting a bathroom in the basement first, before I remodel the two second-floor baths which both need to be remodeled and will both need to be done simultaneously since they share a wall and that wall is moving. We lived through a kitchen remodel that left us without one for 4-1/2 years :(. By having a full bath elsewhere I hope to not have to make any finishes decisions based on timing...we'll see.

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.

Why Sanitary napkins as shown in the photo? Also lotion? Are you going to do a hot water hook up to the tub? I know people who have had hot water hook up outside so they can rinse and wash the dogs before they come in the house. I always bring my boy in and use the warm water and sprayer in the tub. He jumps in and out by himself, does not slip on the non -slip bottom, but one of those rubber mats would work.

Either you have naturally acidic water or your water softener needs adjustment. Pure water has a pH of 7. If the pH level goes below 6.5, water is considered acidic, or “soft”. What a water softener does is increase the acidity of the water. If it is too acidic, however, it begins to corrode the copper in your water lines. As you surmised, it is the copper oxide that is causing the blue stain. If you have a water softener installed, ask your softener technician to adjust the PH so it is at or just under 6. That should stop or at least reduce the problem. If your water is naturally acidic then the solution is more complicated generally involving the installation of a chemical feed pump to inject a solution of soda ash into the water to raise its PH. To clean the stain, never using a scouring powder. Bon Ami was recommended by Kohler years ago because it was the only cleanser then commonly available in supermarkets to use feldspar and limestone (collectively called "rottenstone"), considered "soft" scouring particles rather than pumice, a very hard particle. Still, it is a scouring powder and will micro-scratch the tub's surfaces. Micro-scratches harbor dirt and micro-organisms, making the tub harder to clean next time, and the more your scour it, the harder it is to get clean next time - a vicious circle. Barkeeper's Friend (BKF) cleanser is a chemical scourer rather than a mechanical scourer and is safe for most enameled cast iron. BUT, always test in a small, less noticeable spot on the tube before using. Also be sure you buy the cleanser. BKF makes a specialized cleanser and polish especially for metal cookware that does contain soft scouring particles and should not be used on enameled fixtures.