Shower – Replaced tile grout-soaking up moisture and comes out on the hand

Thinset will normally be 80% cured after 24 hours for moderate indoor temperatures. 80% is enough to do any work on. At 18 hours you got some water on your floor. The water has little to do with your thinset now. What you are worried about with the thinset is putting pressure on it while drying.

Imagine glueing something like a magnet to another piece of plastic with something like gorilla glue. Let's say at the 20 min mark you move the magnet - even a little. Will it be glued as well and hold up as good? No way.

But the water didn't really put pressure on your tiles to move them unless you had a ton. Now you standing on the tiles while plumbing is a different story. However what are you going to do? If you moved the tiles a little and killed the bond then it will pop out on you eventually and you will have to fix it. Unless something is moving on you now there isn't much you could do with it.

For a pure shower (not bathtub), I always put a membrane on over the backerboard. Redgard is my preferred choice right now so I would be fine with that choice.

I really don't think there is anything that you need to do. If I am putting plastic behind my backer I should see it sticking out under the backer and over the base in the picture - it would be covered by the lowest row of tile. It's not... not the biggest deal in the world if you have a membrane plus backer.

Final take: There is nothing wrong with Zhentar's advice. If I were doing a bath/shower combo it is exactly what I would say. However with the use of caulking for all of the gaps I would one a solid membrane over the area. The fact is the caulk will dry out expand/contract and may leak after 5 years or so. You are binding the caulk to larger materials like the DS which will have different expansion rates. With a membrane you have one solid surface that is made not to crack.