[LadyMe]

Update! Problem solved but not fixed.

I found a clog in a most inconvenient place. Not only does this address the problem with my overflow holding fluid hostage but it also solves the problem with my heater as the clog is at the base of the hose that feeds the fluid back into my engine, which explains why the fluid isn't moving up into the heater core from the radiator.

Unfortunately for me, it's not just the hose that is clogged - but the "nipple" it attaches to is also clogged (& curvy & hard to get to). LoL, perfect storm. So... if you are finding this thread and having a similar problem then you may want to check the hard to reach & impossible to look into "nipples" as well as the hoses. While it's still a work in progress the healing process has begun!

UPDATE: The nipple is cleared. I snaked it out with my old choke cable and used a hose to capture the ick and pull it out (one tiny bit at a time)... but it worked. My coolant is makes it round trip through the engine now. My overflow is no longer holding my coolant hostage and I can have a thermostat in without any problems. WooHoo!

Cheers!