Soso
TF
If you have any spare tiles remove one tile to prove it . I watched that rogue traders guy core drill a drive then get the contractor back . When the contractor arrived he said if you cut that drive in half you would this and this . Rogue trader guy said "funny you should say that " and it went from there .
Luckily, he has left some tiles under the sink behind the vanity ungrouted. I can stick almost my entire little finger behind one of them ! Hard to see behind the others though without removing the vanity. We also saw major gaps behind the mosaic to such an extent that whole sections of tile were not secure and he said that it was nothing to worry about and that the grout would seep in behind the tiles and it would all be fine ! But if we can get more evidence like you suggest i think it would be a good idea for sure... Trouble at the moment is that whatever we say, he comes back with a lie to cover himself. Not a great situation.