J
J Sid
Just buy some new tiles
Hardie isn't waterproof, just has a resistance to water. Yes your xps tray is waterproof but you don't want to be letting hardie sitting submerged in water it's the layer between the tiles and tray so it would be. You would want to help the water pass off it and not through it.
If your unsure read the hardie instructions or call them, Its stated in their manual to tank it also in a wetroom situation.
It's the same as saying the tile adhesive under the tiles is submerged in water all the time.
You need the tanking to displace the water that reaches the hardie, not bare hardie which would soak it up rather than displace it. Water will pass through the grout and reach it unless you go down the epoxy route.
In these types of job you are trying to limit failure, for the sake of what it would cost to tank less than a square meter i don't know why you are dead set against the advice?
Anyhoo i've already advised all i can on it. I'm sure others will come along with similar advice
Slc is the route I'd take if I had to fix those tiles