Tile Floor Issues
You need to removed everything until you get to the subfloor which is the 1x6 t&g you saw from the basement. Assuming the joists are in good condition you can then re-fasten and repair any bad boards and then install 5/8" or 3/4" underlayment grade plywood. At that time you will decide whether to install a concrete backer or go with a membrane like Ditra instead.
Do the first step and then let us know what you find and which way you plan to go.
Tile 4 You LLC Troy, MI
KERDI Shower Specialist ... DITRA Installs ... Product-Method suitability consultation. I have NEVER made a mistake, I thought I did once...........but I was wrong! - PRODUCTS ADVERTISED WITHIN MY POST ARE NOT ENDORSED BY ME!