Just a suggestion - you may want to leave the hull fitting and install a plug that you can grind and fair flush in the opening. Later if needed it can be drilled out. This may help if you ever decide to sell the boat to someone outside the Great Lakes.

You should be able to go directly from the head to the holding tank because the tank is vented, and if your tank is like mine, it enters near the top of the holding tank so that there isn't back flow from the tank to the head.

You probably want to barrier coat anything you do, no matter what the approach to keep a good moisture barrier. I just did keel repair and refaired the glass due to cracking and chipping. I put barrier coat over the entire keel and the transition joint just to be safe and prevent water infiltration. Maybe moisture infiltration contributed to the glass cracking over the years on the keel. I consider the barrier coat an insurance policy for any exterier work done below the waterline.

Bill
Rhapsody J/30 #348