it depends on your concrete. every job is unique, so we have no idea how easy or difficult it will be to remove the thinset, plus the amount of residue that may be left. It also depends how thorough the person(s) is/are who are doing the work and their degree of expertise in this area. Anotehr consideration is the type of setting bed and type of thiset used (mubbed or thinset, polymer/latex fortified or unfortified regular thinset). it also depends on your expectations of overall looks as well. if it was sealed concrete then you have a much better chance of being satisfied with the outcome because the thinset may come up easier than it would on unfinished and/ot rough concrete.

Sorry, I realize now that you already tried acetone. Did you thoroughly scrub with the acetone or just do a single wipe? I found a single wipe did not remove the adhesive residue for me and had to do some serious scrubbing. I still suspect in your case that it's residue that has permeated the stone. I never tried just applying sealer (and I wanted a non-enhancing sealer in my case) because I was afraid it would simply seal in the residue and the mark would still show. But with some work, the acetone did work for me. I haven't tried yet, but I had some tiles scraps that sat for a year outside with masking tape. I decided to try cutting them into coasters and they also have a dark mark where the tape was. I intend to try acetone on them, but haven't yet tried. I imagine that the longer the adhesive sits, the more it will seep into the stone.

Do not worry, they will go away after the pool is full and brushed for a few days, even quicker if they do an aggressisive acid start-up.

It is a good observation dadoes. I do not have an answer. Mamapinky, when I finished washing with stpp the whole room had something it did not have before. So, I know that whatever ingredient it is was definitely strong and it was not in the clothes before. I have rinsed the clothes and the washer many times - with TOL detergent, with vinegar rinsed by hand. Nothing has made the clothes or the washer back to their original form again.