Unless yo go to a lot of trouble to rot proof the existing timber and the new cleats, you're looking for trouble.
Get rid of all the old floor.
450DB on the underside is a complete waste. A resin coat is all the original builders (and often, not even that - on Masonite!) and those boats lasted decades.
The spans between stringers is close enough that you don't need to add extra glass underneath for stiffness.
A !/2" (12mm) ply sole is probably enough on its own. The glassing underneath seals out the moisture, the glassing above makes it waterproof, bonds the edges to the hull and makes the sole harder wearing.
Also, when you replace the whole floor, you can honestly say that the floor has been replaced, when it comes time to sell.
It's just good practise and is probably easier than what you are suggesting.