slinky

possibly better to use 18mm ply, screw down every foot,yes bal prime is best.
when butting floor to wall leave expansion gap of a few milli as the ply will swell so use silicone not grout .also best to rework adhesive when checking adhesion,
use rapid set flexi for floor this will cure ready to grout in 3hours,the grout is correct too.prime walls also if using same cement based adhesive.also if porcelain a tile sealant before grouting may be in order,good luck.