justs seems stange to use ply instead of slc and slc instead of ditra think would be a sounder base to just slc it then tile happy days :) think ardex can be tiled in four hours but as advised above i would give it 24
i agree with everything Richard says i would proberly use tile backerboard you are the customer demand what he has told you as not overkill it will insure it lasts :)
some seal before laying some dont but you must seal them before and after grouting, 4mm grout line would be fine make sure they are adhesive, and dust free before sealing