Thanks - ran through some of these to further isolate the issue... including removing both WH hoses and creating a short bypass loop - definitely had good flow from the pump.
In any case - third time was the charm... I managed to borrow a 12V fluid pump, and put a lot of coolant through the WH loop (there was air coming through for quite a while for some reason), then reconnected and that got it. I ran the engine, including under load, at the dock for over 1/2 hour, and never did quite get to 160 degrees.
I was finally able to check how the exhaust riser replacement was working, which was the whole point, and was very discouraged to see a pretty good coolant leak onto the area of the aft port engine mount. Thankfully, though, it doesn't look like it's from the exhaust flange, but from the fresh water return hose connection on the Hx... ran out of time today, but will go back to tighten and confirm that tomorrow (and also investigate why, after running the engine for as long as I did today, I didn't have hot water... ).
thanks again for the suggestions