Lots of people employ builders to renovate their homes. Some builders actually use tilers to tile, plumbers to plumb, sparkys to do the electrics etc. However, some builders think they can do everything themselves. Unfortunately, Indrin found out nearly too late that his builder was the latter. Indrin also found out (too late) how to prep & set tiles correctly.
This builder cannot tile. Do let let him tile in your house. Follow “Timeless Johns” advice.
This builder does not have a plausible excuse for what he has done to your bathroom. Do not believe anything that comes out of his mouth. You can print off this thread, but he will not look at it. He will say he has been tiling for 20 years and he has never had a problem.
You might be lucky and save a few tiles.
Good luck.