Good questions and ones that I considered for this repair. No offense taken. When I repaired the port side bulkhead, I replaced the entire bulkhead in a traditional manner as shown in the following link.
S/V Victoria Head Reconstruction
For the starboard side, the rotted area was small (6" square) so I elected to insert a panel and tie the repair to the old and new sections as a way of distributing the load. It was a calculated decision since the shrouds to which the chainplate is attached are the forward lowers and under far less load than the uppers. The fact that I'd have to disassemble 2 cabinets, an A/C unit, and shelving also played into the decision.
I cut well into good wood. The replacement panel is scrafed with a rabbit. I would have liked a taper but it was not possible since I was working in a closet. The panel is joined to the existing bulkhead with thickened epoxy and a layer of fiberglass cloth which is only there for lateral stability.
You are correct that the deck provides counter resistance but I didn't count on it since the panel extends outward nearly to the hull (1/2" gap) and is tabbed to the hull and well down into the existing bulkhead. For tabbing, I used roving and not
cloth. When I tensioned the shroud, I watched for any distortion. If I saw any, I was prepared to replace the entire bulkhead. Not only was there no distortion, it didn't even creak. After a full season in wind up to 20 kts, there is no distortion or slackening of the shroud. I'm confident of the repair but if I detect any compromise, then the whole thing will come out..... it's not like I haven't done this before! It's only time and virtually no money.