You're already scratching the surface: vertical integration, backdoors and high prices, power and battery life optimization, lesser effort for an app developer to publish to all platforms, develop-once-run-everywhere, ...
Looks like you're looking for one "The Reason" - but there doesn't need to be one. If a layperson like you or me is able to provide 5-10 reasons, then its likely there could be 100-1000 reasons internally, and all of them add up.
> 2) iOS apps on OS X. Why? Does anybody want this?
Of course yes. As an app developer and as a consumer - convergence and bringing my apps and data across all platforms is no longer an "optional" thing anymore, its mandatory even for a ToDo list app, or email, or IM and everything else. Web apps suck at power efficiency - see the situation with Slack/Electron/Chrome/others on desktop, especially when it comes to stuff like hardware bound work (video/audio, digital image and movie processing, hidpi wor and much more).
If you don't need all this, and all you need is just a chromebook with a browser, its fine, it has and will keep working. It also ties in to why you're confused "why this is required" in so many ways. You may not be the target audience here.
> 3) Given the hellscape of bugs currently present in iOS/macOS, does anybody have faith that Apple is going to be able to navigate a rewrite of macOS on this scale?
This one answers itself. If a fragmented platform doesn't work and has lots of bugs - then it makes all the more sense to invest all resources in one platform/arch to have better focus and lesser bugs to tackle. Everyone doing rewrites know that there will be short-term pains, but that has to be balanced with the larger picture - otherwise we'll just keep hating new releases but there will be no solutions other than "let's do only bug fixes for next 1-2 years" aka platform stagnation, and users still won't be happy :)