Your engineer was basically saying that you have a high water table, meaning that the underground water is very close to the surface of the ground.

Xypex is suppose to waterproof the concrete. So in theory it should prevent moisture migration in the concrete wall to help reduce the possibility of efflorescence.

Some types of natural stone will tend to warp when they are subjected to moisture and then dries, and when they are subjected to high temperatures and then cool.

I’m assuming the hollow area under the tile is a void.

Exterior decks are one of the most common failures in building construction because they are normally constructed incorrectly.

Leaks sometimes are not easy to find. They can show up in one spot, but the source can come from another area far away.

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The leaking of deck causing damages in the ceiling below is the symptom of the cause of the problem. Treating the symptom will not remediate the cause of the problem.

If the tile was installed correctly, it should not be damaged by being submerged in water. Although quite often tiles are not installed correctly, and there can be resultant damages from a flood.

I assume that the concrete slab is an interior application. Generally speaking you don’t what to install tile over a concrete slab that has a hydrostatic water problem unless it is remediated first.