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Last November I did my OFHG and most of the coolant got changed as part of that job. I purchased a vacuum filling apparatus from ECS I believe it was. I'm pretty sure I have an OFHG post around here somewhere and I briefly cover it. I was never even tempted to do a "flush" - too many ways to get the coolant mixture out of whack.
My goal was drain and replace as much volume as I could reasonably do. If you don't get "behind" on coolant changes, it's not even super critical that you obsess over getting as much as possible out as it is all "fresh enough".
1/ Although it was $80-$90 and I would have preferred not to be required to buy the vacuum filling apparatus, boy was it slick. Ten seconds on shop air to draw a vacuum, however long you want as a leak test (nice to know all of your fittings are secure as part of the job), and something like 5 seconds to draw all of the replacement coolant back in.
No bubbles, no burping, no top ups for the next week, no guessing. Combined with the built in bleed program it was, simply, the best coolant fill and bleed process I've ever done.
Now that I have it, I'm planning to use the vacuum gear on any car I do coolant on, not just cars which require it.
2/ If I recall past learning properly, the water pump is never supposed to be exposed to air. Use caution that the pump always has some coolant in it while going through these processes and DEFINITELY do not let it run dry.
*** I do not recall if 100% water is bad for it either because of lack of "lubrication" of the pump seals. Water is better than dry, but it may actually need coolant mix if it is being asked to run. I'm sure someone else will chime in or google would be able to answer the question.
3/ Again, combined with the vacuum filling apparatus, I think the bleed program does it all. I think I ran it twice while connected to a beefy battery charger just to be sure. There was no change in expansion tank level before or after the second run, only the first, implying that one run would have done it all.
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