Advice on dealing with a faulty contracting job

a more pragmatic suggestion would be to treat the interior like an exposed basement wall (but on all sides).

you mentioned a ceiling water barrier, but i didn’t see the details; obviously this is not working. i would run an interior dimple mat, applying a single sheet from one side of the floor, across the ceiling, and down to the other floor; overlapping seams 6” at least with ridiculously generous amounts of mfgs sealant. this will dump any water on your floor; hopefully there is a drain or other means of disposing.

add furring strips, 1/2” or 3/4” xps (butted with foam caulk). This will give you a moisture and vapor barrier for your preferred interior finishing.

personally, i would use pvc trim boards for wainscoting (actually, because ceiling is always a risk i would do pvc everywhere), and do your painting and finishes off that.

the weather is getting too extreme and we need to start thinking about when things get wet, not if.