Dang, the back problem is a real bummer. Hope he feels better soon. Keep planning, it's fun even if you can't tackle the project just yet. Besides you might find even more cool ideas in the mean time.
If you do decide on glazed tiles, another way to ensure you get high fire is to go for files labeled porcelain. Tiles can be very inexpensive if you shop well. Not sure if you have Floor N' Decor there but they always have nice neutral glazed tiles in the $1+/-ft range. I think you could probably find something at Home Depot or Lowe's too.
I think in a freeze area like yours I would still consider using a vinyl caulk instead of grout for tiles. Reason being that grout will absorb moisture and then the freeze causes it to crumble. The flexible caulk will offer more expansion for the surface and keep the grout lines in decent shape longer - or at least it seems logical that it would work that way.
Looking forward to hearing more about the backyard kitchen
