A BeOS-based MacOS would’ve been ready for prime time years before Mac OS X was.
I'm not so sure. NeXTSTEP was pretty stable. I get the feeling that a lot of the delay had little to do with NeXTSTEP and much more to do with Apple and Adobe/Microsoft. Plus, the development environment was no where near as good on BeOS.
I don’t know. A BeOS-based MacOS would’ve been ready for prime time years before Mac OS X was. And one suspects the Finder would have been usable. But there would be no iPhone.