To answer your original question, whether you can save the grout damaged by low pH . . . probably not. Once the cement & calcium binding the sand has been dissolved, you can't 'un-dissolve' it. You might be able to cover it over with something that will hold it together for a year or so, but I'm only guessing.

One further note: I think it's unlikely that your grout was destroyed by a single winter of low pH water, unless it was VERY low. It's more likely that this deterioration has happened over time.

So . . . I would STRONGLY recommend that you purchase a K2006 (Amazon links in my signature) and learn to test your water accurately, and manage the pH and alkalinity properly BEFORE you do any repairs. That way, you won't have to worry about the same thing happening again.

Sorry for your troubles.