If you are doing the floor like that...I think you could get some "Get-Rot" (a marine product) follow the directions and I think you will be able to stabilize the floor rot problem. As for leveling....get some leveling compound...used under vinel tile flooring....but you can use it for what ever. Easy to use also. Be sure to kill all that mildew you have mentioned elsewhere. What floor covering will you use?
"If it can't be reduced, reused, repaired, rebuilt, refurbished, refinished, resold, recycled or composted
then it should be restricted, redesigned or removed from production."