Your plumber should have checked with you which toilet you are installing and asked for the specifications before he roughed in for a bottom inlet (tap at the side of the pan). You should call him back. Most new installations use toilets with a back inlet nowadays (coming in through the cistern as your toilet does) so he should know better.

big reader Im chasing up inwall toilets now. I've read a couple of posts and your grievance with your in-wall keeps cropping up. Would you mind please elaborating on why yours was so bad? What brand, what went wrong?

we have two in-wall toilet cisterns, one built into a tiled wall backing onto the tiled shower recess and the other built in under an extension to a vanity...so far no problems but they're only four years old and were only new on the market when we bought ours ...the manufacturers' website had detailed structural diagrams and installation instructions and you'll see that the tank is sealed from any foreign objects (including toilet cleaners and inquisitive children) and one model has a cover plate with the flush buttons that can be removed and is wide enough for access and the other has the flush buttons directly on the tiled wall...they are surprisingly basic for the extraordinary price compared to a standard toilet cistern but, combined with the wall hung toilet, we also think they look great but we made sure to keep an extra metre of wall tiles "just in case"....another options we considered was a remote cistern mounted in the roof space with the water delivered by a pipe in the wall..... hopefully since we installed ours any faults have been identified and resolved and the latest models will be even better

No personal experience with these systems but I know they do make special acoustic rated gyprock style wall boards for this kind of situation. And acoustic paint coatings too of some sort. Would they help?