Regardless of what type of tile you install over your garage, none of the installations will be waterproof unless you waterproof under the tile first.

It is possible to embed 12×12 inch ceramic tiles into a freshly poured concrete slab, but there are a lot factors to consider. It is recommended to let the concrete cure for 28 days before adhering the tile to it.

The first question is why did the tiles come loose? If installed correctly tiles should not come loose.

You might have more than just a lack of maintenance problem. If you have mold on the tiles, then the tiles are being subjected to excess moisture.

You might have a cementitious grout haze over the tile or a latex residual haze from the grout or thin-set adhesive. There are products for removing those types of conditions.

It is always wise to control dust for both health reasons and for cleanliness reasons. Installers typically use a wet/dry vacuum to collect the dust as they cut through concrete or mortar.

First of all if it was installed correctly you wouldn’t have a problem. To fully figure it out I would have to perform a forensic intrusive inspection, which probably isn’t practical for a small shower.

For an ADA shower you can’t have a curb/dam. You need a minimum slope to drain of ¼” per foot and no more than ½” per foot. You need a two part drain with weeps for the primary waterproof membrane to clamp to.

Hollow sounding tile is not a defect per industry standards.

Large rectangular tiles tend to have warpage at their corners and at the center of the tile, so when two adjacent tiles are off-set from each other then you can get compounding warpage.