Looks like you've put a lot of thought into it. That will make the job go easier. I've got that same oscillating tool and it's become a go-to for sure. Spend the $25 or so each on the good carbide tipped blades and they will last quite a while. The stringers are probably wet because the factory left the holes on the tops open where the poured in the foam. They figured the deck on top would seal them up, but once the deck gets wet the water can get in them. Unsealed screw holes were a factor too. They could also be delaminating somewhere.
