Good thoughts to consider, John. In my situation, my head intake is closer to center line than the sink drain seacock. The head intake is also underneath the vberth -- while access is not difficult, it is not convenient (need to move a vberth cushion and raise a panel. My plan is to not even open the head seacock (unless fresh water stores are low), so the chance of any backflow is unlikely or impossible. Fair warning regarding sink debris, but the head sink is used pretty much only for hand/face washing.
Unless I'm missing something, I think the concerns you raise are more applicable to those who use the same thru-hull for sink drain and head intake.
Okay, let me see if I've got this straight now:
Your goal is to have a freshwater flush for the head. You will get the fresh water for flushing the toilet from the head sink. You plan to "T" off the sink drain, and run a line to the toilet pump intake. You will run the faucets in the sink whenever you need water for flushing the toilet.
You will no longer be using the salt/lake-water intake for the toilet. That seacock is not easily accessible and will remain closed pretty much all the time.
Therefore, you will not be using the head intake seacock as the sink drain. Instead, you will continue to use the sink drain for its intended purpose.
Okay, now that I've walked myself through it, I see your plan better. Something in your initial post led me to think you'd be doing it a bit differently.
The remaining question is about the need for the diverter valve at the "T". Without it, you will have to close the sink drain seacock and first fill the drain line with water before it will divert at the "T". So, depending on how long that drain line is -- you might want the diverter valve. Personally, I'd install it.
You may still have issues when the boat is heeled with being able to operate the sink faucets when you need flush water. Gravity may conspire against you.
Again, this is where the set-up that Christyleigh and we have is in many respects simpler and more fail-safe. What we do is we grab the combination shower/faucet head (which is on a 6-8 foot retractable hose) and simply spray the fresh water in the toilet bowl directly. (With the added benefit that Christyleigh pointed out of a pressure wash!)