Miles, fan running 1/4 speed in heavy traffic = overheating. Fix alternator and re-check for overheating. Might also do a fast charge on the battery before setting out.
Did any of this start right after KCM fixed your busted rod from the Hallett episode? So engine has been opened up .. what got diddled with then? Did the intake manifold come off? If so, then a water passage may have been fouled/blocked.
Can you see water circulating if you let the engine idle with the radiator cap off? Start this with a cold engine and a known good t-stat. No water circulating = broken water pump impeller, or sheared pin and it's spinning freely.
How about the timing? Way off timing can cause engine to run hot.
Coolant mix is probably not causing this much overheating in the current mild autumn climate. Something is broken. Good thermostats sometimes go bad, easy to change. Can also test it by dropping it in a pan of 180 deg (or whatever temp is stamped on the plug) water (use meat thermometer) and see if it opens up. Pick it out of the pan with tongs, not fingers
My problem was a bad water temp gauge, not the t-stat. But I went through the entire menu of possibilities before I discovered that, including changing the thermostat and coolant.
Think .. when did the overheating problem start? What changes immediately preceded that?
Call anytime for in-person consult
Sam