After many hours of work, and one hiccup after another, finally flushed out the system. Not much rust, but some dirty old coolant did come out. A little more dirt is seen here because coolant first fell on the grime in engine bay and then collected in the tub.
During the course of this DIY, I noticed the O2 sensor wire was hanging on just by one copper strand; snapped it off, took out the three joints within 3 inches of wire that electricians had put in there, and soldered the pin on the wire directly. Same happened with the TPS vcc wire a few days ago. They both snapped near the grip.
In the end, used some diluted sweep to clean out the reservoir before putting in Zic's antifreeze.
Does antifreeze have a shelf-live?