Extremely poor working by the work man. I think you should needed to do with expert then all your issues could solved. We recommend you to do work with great and expert persons rather then inexpert person.
I used Silicon matched to my medium grey floor tile grout on my quad shower / floor joint rather than white to match the tray. Makes slight variances in the gap lot less obvious.
12mm is not an acceptable gap to try and cover with Silicon! I think from the photos we all know it’s not been tanked, and it’s almost certainly had underfloor heating fitted onto ply wood and then tiled. Tanking isn’t mandatory so really you can’t pull them up on that.