Sigh. We were not aware that we were supposed to check for defects after hacking and before they install the glass, kitchen countertop and tall unit. Our ID also didn’t advise that such defects could have happened. Naively thought that defect check could/should only done after everything was done, so painful lesson learnt there.
Spoken to another ID who concurred with what you said, that nothing could be done except to overhaul the whole setup, which is probably too costly and time consuming.
As for the black marks, the ID tried thinner but it didn’t seem to work. The second ID said the marks could have already penetrated/seeped into the tile.
Meeting the ID and his supervisor this weekend, so let’s see. Probably seeking compensation but we’re not sure how much should we be asking...