I just did a kerdi shower ceiling with 12x24 tiles. I used unmodifed thinset per kerdi directions. I used laticrete 317. I know versabond will work (but its lightly-modified) and would void the Kerdi warranty if used. The 12x24 will stick just fine. It's no different than doing a 12x12 because even though it is twice as heavy, it has twice the surface area to hold it. I did back butter each tile. When I tried to pull one down to check coverage it was really stuck up there! My biggest problem was that I started tiling from the center to the sides and i have a slanted ceiling so the tiles wanted to slide out of postion. if I had started at the wall and used spacers it would have been easier.

Has the tub and Green board been installed correctly? Shove on that board and see if any fasteners pop and add fasteners if needed. Check with a long straight edge and look for humps. Hollows are easy to fix but humps prove to be problematic and slow a fine tile install. Brush the Green board with a dry paint brush and shop vac and remove any dust. Insure all seems are taped and reinforced. You don't want any voids and Hydro Ban wants nothing more than a 1/8" gap. Give your shower the best chance by starting with a sound, void free substrate for the waterproofing. It takes no time to fill all voids and delays the waterproofing date by a day at most. If your tub was not filled and drained 10 times back to back after installation do this now and inspect from below if possible. Camera snakes are great for this and offer huge piece of mind if access from below is not possible. It is crazy not to test these drains before waterproofing and setting tile. This is not an option. Fill and drain that tub....

It sounds like a simple tub and caulk problem, not really an issue with the Kerdi. Do you recall if the tub's flange was fastened to the wall studs? Normally a nail goes through the flange into each wall stud. That'll prevent some vertical movement/deflection at the flange. I can't say if adding under-support will help at this time. If you hear squeaking and creaking when you're in the tub, that could be a sign of movement. Do you see any movement at the flange when the tub is loaded and unloaded (water, bathers, etc)? Or do you think that the original caulk failed simply due to a one-time "after installation shift" and that the tub is now settled in place? I'd pull the existing caulk. Clean, let dry. Load the tub. Then install silicon. As far as the mold returning, realize that bleach will only kill the surface mold, but not the mold's "roots", so the mold can grow back since it was never killed in the first place. Household bleach is about 6% "bleach" and 94% water. When put on grout or wood, due to its polarity the bleach stays on the surface of the material. It doesn't penetrate into the material. Only the water penetrates. So when you bleach a porous material, you are indeed killing the surface mold, but the water is feeding the mold's roots. Not a truly technical description of what happens. But that's why bleach itself isn't an approved "mold killer".

I'm not a fan of Custom Blend unmodified, but if I have to use it, I'll buy a bag of portland cement and just add a trowel of portland cement to each batch of Custom Blend I mix up. It enriches it a bit.It still meets the criteria as being unmodified. Versabond? I've used it. And I've had no issues with it. It's lightly modified. The studs, I agree with a screw hitting the seam between two studs sandwiched together as not being an issue. For framing shower doorways, I screw the jack/king/trimmer studs (whatever they are called in your locale) together with deck screws.