On all of the mastic buckets I’ve looked at, there is a maximum tile size limitation, so unless there’s a new one out, mastic is not approved for a tile that large. The reason why mastic survives in a bucket is because the lid prevents it from drying out. Now, some moisture will be absorbed by the cbu, but until that happens, it will remain flexible, and tile doesn’t like when it can move!
Something like RedGard is not a requirement to meet industry standards in a tub surround. You do need a vapor barrier behind the cbu or waterproofing on top of it, but not both.
Sealing the joint between the tub and the walls is most easily accomplished by running the vapor barrier behind the cbu, and lapping it over the tiling flange, then leaving a slight gap between the bottom of the cbu and tile to the tub surface and caulking that.
Wall to floor joints are expected to move, and only need to be waterproofed in a shower.
Neither tile nor grout is considered waterproofing...they are the decorative/wear surface. A wall in a shower does not need to be waterproof as most all of the moisture is expected to just run down the surface (hopefully, unobstructed!) and to the drain, and what may penetrate slightly, should dry out in between uses. A shower pan, though must hold water and be waterproof, but again, that’s not the tile.
When installing the cbu on the floor, did he use any mortar underneath? All of the manufacturers require that (and mastic doesn’t cut it there!). if the panels were just screwed to the floor, that’s against the manufacturer’s instructions and would be a reason to require fixing. Tearing up a large tile like that can be messy, and you may not be able to salvage much of anything. If you do, the mastic will wash off if you soak it first, scrape off the softened mastic, and scrub it clean. The labor to do that may mean it would be cheaper to buy new tile.
While mastic is approved for use on a shower wall, none that I”m aware of are approved for use on the pan of a shower, but it sounds like you’re talking about a tub alcove, so mastic, if the size of the tile was within allowances, could be used. Generally, it ends up more expensive than a decent cement-based thin set, but that takes more time and cleanup since you can’t just put the lid back on once you’re done and there’s no mixing.
Check out
www.johnbridge.com for help with tiling.