If I were to guess, and at a long distance away, that's all it is...
There was a long-ignored leak around the plate on the cabin top that the spar sits on. This plate should have been removed and the holes epoxied and then properly redrilled & resealed over a decade ago. I have seen this cause problems on many makes of sailboat over the decades.
Pull all the staples, open the zipper, and tie the headliner gently back from the area. I wonder if the bulkhead top is glassed in?
Since you have found rot at the chain plate attachment also, I can see why you need to restore the strength of the whole bulkhead.
While the chainplate(s) are off -- and be sure to do both sides -- epoxy in the deck openings for them and recut.
One other thought (and suggestions from over here in the peanut gallery are never worth too much) is that you fix the parts of the bulkhead that need it, and leave the other 85% in place. Once the rig is off the boat, remove the mast post and the chain plate. Following the standard of scarfing in replacement pieces with a 12 to 1 scarf, install some pieces of G-10 frp, also scarfed to fit. You could buy single-thickness G-10 or laminate a couple of pieces of 3/8".
Once you finish glassing, this will leave some layers of biax visible on the bulkhead. Last, cover the whole bulkhead with a new surface of ($$) teak veneer.
This repair should finally be pretty much invisible, and that's what to strive for. Close as you can.
Sidebar: While it kind of hurts my pride in my work, all my
best maintenance/upgrade/repair work on our boat is 99% invisible, even when you know where to look...
Additionally, this winter you could buy dinner for Tim and David and have them come over and help scope out the project...
Thanks for the pics, and please keep us posted.
LB