slats11 - great post.
I think the South Korea data is probably the best for looking at CFR (circa 1%) - they have done the greatest level of testing as far as I can see and also have one of the lowest CFRs. So either a) they have a best treatment/health service response, or b) they have weaker "strain" of Covid (2 have been identified but no one seems to think one is more or less virulent than the other) or c) they have a much better (and higher) count of overall cases as they have been testing the most people?
This sort of assumption also works for the current UK CFR numbers that to me look high as a proportion of overall cases but we've only been testing folks who have presented at hospitals with significant symptoms.
There's no way to prove any of this is true/false at the moment as we don't have apples or oranges to compare. That being said your analysis looks sensible to me.