Excellent writeup. I just have a question though before I go through this trouble.
My wife's car, 2005 GP v6 (non-s/c), has a heater blowing cold air. We've done two complete radiator flushes. I added the green mixture instead of DexCool the second time around. I've replaced the thermostat and have checked all of the hoses for leaks. The hoses going to and from the heater core are very hot. When flushing the radiator, both times, I flushed out the heater core as well. Once I do a radiator flush, the heater works perfectly. About a month later, the issue comes back.
Do you think this fix my problem?
