Glues just won't work - limited load carrying ability, even more limited shock loading resistance, plus glues are dependent on the load carrying ability of the materials you are bonding to (and PB is pretty weak). Whilst plasterboard carry quite a deal of weight if the load is spread over a large enough area: I have bookshelves in my study/office/work room which must be carrying over 250kg - but that load is no more than 250mm off the wall, and the load is spread over more than 50 fastenings - some of your items have 2 or maybe 4 fixings, put a lot more than 5 or 10kg load on each fixing. If the majority of the weight is being carried on the floor, you have no problems, it's where you have weighy with can cantilever off the wall, like an unsupported shower door, that can cause problems
TBH for the heavier stuff, not soap dishes or small bathroom cabinets, bur cisterns and towel rails, on a new build or conversion we'd take the hit in snagging, cut one or more tiles out, plus the PB, fit a plywood pattress and make good. Less risky, no call backs. Personally, I think this might be something your builder should be doing - he should certainly have asked where stuff was going and what it was you intended to fit
