Unsealed flooring next to shower and strong smell of mold
The fact that you can smell the mold is pretty alarming. I've seen some fairly bad mold in bathrooms and I don't recall it smelling particularly horrendous. The big problem you've got is that you simply don't know how much mold is there (I'm guessing "a lot") and that it will NOT go away by itself. In fact, it will simply get worse and eat into more and more material.

I simply can't see any other option than to start removing tiles ASAP to identify where exactly the mold is (I.e., under the shower tray or under the riles) and then looking to replace or repair as appropriate.