And I think your assessment is too pessimistic.
To address (1.) You and others are going to have to provide sources for why you think that China "probably has under-reported by at least a magnitude if not double that". I've seen the ill feeling and distrust towards China increase on HN and elsewhere in the last 5 years and this fits the bill.
You can explain the differences between Hubei and Italy by the differences in responses. As has been pointed out, China built two make-shift hospitals in ~ 10 days and quarantined 100s of millions in cities. Have you not seen the pictures coming out of China. Now that Italy is taking drastic steps we'll see the outcome in a few weeks and it will tell us if it is an effective strategy. If any of the reported numbers is suspect it is the total number of cases that the USA is reporting: https://www.worldometers.info/coronavirus/ It's completely out of whack with every other country. This implies the US has been under-testing, something which we have lots of evidence for. On the other hand we have no evidence China is under-reporting.
To address (2.) "Most Western countries won't impose the type of strategies that China undertook; they simply can't." Why not? Why can't they?
To address (3.) "Most of the world doesn't have good healthcare." But most of the world has soap, most of the world understands what it means to self-isolate, most of the world can implement quarantining. "If the pandemic keeps growing exponentially with a doubling period of 4 days," Looks like mitigation strategies slow the doubling period to three weeks, why pick the worst case scenario?
To address (4.) "Simple math." Yes! Times an unknown mitigation factor of ? This outbreak is scary because it's exponential (highly lethal and highly infectious), but the total fatalities is also highly susceptible to interventions exactly for the same reason.