get him back an show him what you are talking about with the level? you might be too fussy mate , it might be a simple problem to overcome its hard to say without seeing it just remember your tiled surface will mirror your untiled walls
why did you plaster it? you should of just left the plasterboard far better surface to tile, if you are using adheisive to hide 10mm i feel you are asking for trouble
overboard with hardie backer-board trowel over flexy adhesive to fill the voids then screw the board down ( where to screw is marked on the boards) then tile and defects (bounce) in the floor must be sorted first
grout after 24hours unless says different on adhesive instuctions then you are suposed to wait 2 weeks before using the shower i take it is going to be tanked or have you used backerboards ?
you could try this http://www.ebay.co.uk/itm/NEW-TOOL-MARKET-TILE-REMOVAL-TROWEL-NO-WALL-TILE-DAMAGE-/360443501478?pt=UK_Hand_Tools_Equipment&hash=item53ec1b5ba6
but i feel it would be quicker and proberly cheaper to change the board :)
i use ultra but there are loads of good ones mapei (not the one from b&q) bal, webber or even topps tiles own brand stuff is made by palace and is not too bad if you are grouting in white try and use a white adhesive.