I had the same thought as Bill, but now that you explain the countersunk backing plate, that sounds perfectly fine, so you are on the right track.
Going through these things before they become a problem is so much better than people getting hurt trying to save the port side halyard winch from going overboard just before a douse - just sayin! ;-)
The backing plate for a clutch is considered by many to be overkill as long as the area of the holes has some non-compressible material added in there such as what you describe. A small countersink on the top to create a seal with non-silicone based caulk is advisable as well.
You are spot on about two different sized holes in the deck to handle the increased diameter of the nuts as well. Alternatively,
Barrel Nuts with thick stainless fender washers serve well in lieu of the backing plate as long as the structure has a combination of CS and
G10 tube or something like
G10 plate.
Cheers!
