SOme tile have a squarer edge than others. And, overdressing/washing the grout can make the grout joint to become recessed during installation. If you are real careful about cleanup when installing the grout, it should be fairly flat.
Adhesives, especially those with solvents in them, could really mess with the membrane! And, I'm not sure you'd like the cedar there anyway. You obviously can't nail it up as you'd compromise the water/vaporproofing. And, no, thinset is unlikely to make a permanent bond with any dimmensional wood. Maybe an epoxy thinset if you really want to go that way. Check with the manufacturers (both Schluter and the epoxy thinset) to see if they'd approve that.