I addressed the main blukheads in my boat 2 seasons ago. On mine, it was the port side that was rotted, which required taking half of the head apart to get at. Because the wall enclosing the head is part of the bulkhead assembly I did not completely remove it. The bulkhead was a sandwich if 3 pieces of 1/2", one of these being part of that wall. I relaminated two new pieces dto that and filled the rotted are of the remaining one with epoxy. I dug out rotted balsa out of the side deck around the chainplates and filled with solid epoxy. I also tabbed the main bulkheads to the side decks as you described. I agree, it should eliminate a lot of flexing and moving in that area.
Hi all I am in the process of replacing the starboard main bulkhead. I just took possesion of hull #50 in February and have a long list of tasks. I have attached a picture as it sits ready for the bulkhead to be installed. I will do the final fit tomorrow and then seal the edges with WEST. It will be set into place and screwed to the locker then glassed in in two stages.
In removing the old bulkhead that was in really bad shape, we noticed one of the primary reasons that it rotted was that the top of the bulkhead was not sealed and water could easily soak down the laminate. Also they drilled down into the centre of the plywood so they could run the wire for the cabin lamp. This provided a place for water to sit. I cut out the sub deck around the chain plates and found only a small area that had rot. All wet coring was removed and replaced with a foam core. You can see the repair in the picture. We will be glassing the side deck firmly to the top of the bulkhead which should keep the chain plates from moving relative to the deck so leakage should not be as much of a problem.
You certainly want to give this area attention so hopefully you do not end up with this job.