Re: The UI issues
I'm sure Microsoft hopes Cortana will encourage more people to consider Windows Phone-- but there might be more to it than that.
At least some online chatter states Cortana will eventually become cross-platform, extending not just to the rest of the Windows ecosystem, but also to iOS and Android. If Cortana is actually better than Siri, a cross-platform version is interesting; just as Google is ostensibly trying to use Chrome to put a Trojan Horse inside Windows 8, Microsoft could use Cortana to put a Trojan Horse in iPhones. If Cortana is tied to a Microsoft ID, that encourages use of other Microsoft services-- calendars, documents, OneDrive, Skype, etc. I think Microsoft can live with the reality that a lot of people are going to continue to buy iPhones-- as long as Microsoft services are running on most of those iPhones. I suspect Cortana will be WP-exclusive for a while, just as Office has remained exclusive to Windows tablets. But I also expect Nadella will be more aggressive than Ballmer about pursuing cross-platform opportunities.