Hi,
I only have a moment, so I will be brief.
The deck core has nothing to do with the chain plate pulling through.
Look inside your head locker above the sink and you will find the chain plate bolted to the main bulkhead. It is a large piece of metal with at least 8 bolts.
If the chain plate is actualy lifting up, then you will see the bolts pulling through the wood/fiberglass section of the bulkhead. This will get ugly.... To determin the extent of the rot take an ice pick or small screw driver and start poking all around the bulkhead. Don't wory about dinging the wood, you are most likely going to be doing lots of wood work. Poke the entire bulkhead including where it contacts the shower pan. Water collects here and rots the wood.
I had to replace the entire bulkhead.
The ultimate resone there is rot damage is from water entering through the deck at the chain plate cover. In our boats this cover is small and keeping it well bedded to prevent water entry is easy to overlook.
It is very likely that the rest of the smaller bulkheads that secure the forwad lowers have some damage too.
Depending on the extent of the damage the repair will vary from total replacement of bulkhead to treating the wood with penetrating epoxy and then glassing over the entire thing.
I did a good fix on the main bulkhead and left the others for fixing in the near future. As it sadly turns out, my forward chain plate broke durring a coastal voyage near San Francisco and the mast went in the water. I had to cut it loose and lost the whole rig. I was sailing into the wind when the fwd chain plate broke. At the moment the chain plate broke the load went to the port forward lower chainplate and bulkhead. The weekend state of this bulkhead resulted in it quikly failing and the mast went down.
I think if the forward lower did not fail so quikly then I would have had the few seconds needed to sail downwind and put a halyard down on the bow cleat.
Anyway this is a long way to say "don't go sailing without fixing the whole rig. "
As far as the goo between decks goes, leave it for now. It is an easy fix. There are good articals about this process in the archives.
Get a better look at the thing and we can chat more!
Bill