Just a quick update. New filters arrived so I got around to doing one more flush of the distilled water and then replaced the coolant filter. I hooked up the rear heater core and I went ahead and put the GX-05 coolant in so I can finally stop worrying about parking outside in the cold.
Everything is working, but not getting any heat out of the rear core, so I am slightly concerned that the filter is reducing the line pressure too much for effective rear circulation. It may just need a few cycles to purge the air out of the system so I haven't given up hope yet. Front heater core is working great and now I am on to the next gremlins with this car (heated front seat not working).