...that would be me.

You can file this under "Things NOT to Do to Avoid Constant Overheating"

Do NOT inadvertently leave your engine compartment bleeder valve open
(not even a little bit) while you drive.

Doing so will prevent the system from building pressure, thus raising
the boiling point of the coolant and causing all sorts of problems, like
constant overheating under load, a maple-syrup smell you think is caused
by your rear heater (mine's actually fine) but is in reality coming from
coolant boiling over and exiting the bleeder and overflow tank (and
coating the back of your van).

Ask me how I know...

Also, do NOT assume your expansion tank pressure cap is good, just
because it looks good. I removed mine and noticed that I could
effortlessly suck and blow through the hose connection, indicating the
cap is not capable of holding adequate pressure. A brand new cap
purchased yesterday at a local shop (Thanks, BenT) confirms my
diagnosis. It takes some effort to suck and blow through it.

So, with these two remedies performed, I again filled and bled the
cooling system. I bled the cooling system at the radiator so that just
coolant flowed out, no air. I left my van idle in my driveway for more
than 1/2 hour, and the temp rose to just above the LED and stayed there,
not moving. The radiator fan cycled on and off as usual. Drove the van
last night and the temp needle stayed put at just a hair over the LED. I
will report back after my next freeway climb up a long grade, but I
suspect that I have solved my problem for now. Both the open bleeder
valve and the weak pressure cap were contributing factors.

Hope that my experience can save others from a similar fate.

Dan