If it helps you feel less stupid I paid someone to make some pipes up which I'm now going to ask him to cut off to make it easier to tile over. Then will need to be made back up again...Can't find the original retailer link any longer but it's this one:
Sounds like consensus is that I can't chance my arm with adhesives and I should go through the bedroom behind and make a proper fixing.
Very sensible, goddamnit. What a stupid ******* mistake with all the measuring and planning.