A lot of advice there overnight - thanks everyone. I will definitely be adding some more noggins to the studwork around the shower cubicle when I pull the old boards off, better to slightly over-engineer for a tenners worth of timber.
3mm spacers it is then!
I found the guard for my Plasplugs DWW100, so I'll just treat it to a new blade. It coped with cutting my Porcelain floor tiles in the kitchen ok.
Still a tad confused about the latex issue though - does it really aid adhesion or water resistivity (they're supposed to be impervious <0.02% porosity) for these type of tiles, or is it just the extra flex (which most of you think is a bit overkill)? When I put a few extra noggins in, this should pretty much eliminate wall flex anyway. If it's all about flexibility or water resistivity and not about adhesion, maybe just latex up the adhesive for the shower cubicle, and not the general bathroom walls that will go behind a toilet and sink?
Still thinking about the glitter grout, not sure whether thats too much sparkle (if it's not a good match to the sparkle of the tiles) or whether it'll make each wall look like 1 whole piece rather than a series of tiles (good)