"C should be the supply
D should be the return"

Meaning the coolant flows FROM the engine at "C" to the heater core and returns to the engine at "D"?
Have Igot that right?

The reasoning behind my question is trying to figure out the best way to do an absolute thorough flush of the engine, heater core, and radiator.

It looks to me then that by disconnecting the upper radiator hose at the radiator AND running water from a garden hose into the radiator cap, while the engine and heater is running of course, I'll be flushing the whole system. Yes?

I've been running a cooling system cleaner for a few days now and it's time to get all that crud out now.