That assumes a lot from non-technical users.
Not sure why you brought up skeuomorphism. It's poor aesthetic taste but it doesn't necessarily make the UI difficult to use.
"It's not your mother's phone"
It's a nauseating commercial IMO... which isn't to say it isn't effective.
(2) I think you are biasing your opinion too much toward the first-use of an application. Yes, associating icons and labels does aid in learning and first-use, but has little effect on usability long term for regularly used applications. With that said, the in-app icons you are referring to are typically straight forward (add, search, remove, etc) and, in my opinion, are understandable on first use as well. There are occasionally poorly designed third party apps that do have obscure in-app icons. But, this is not typical of the platform.
So...I'd say ease of use is high as long as you only want what is on your home icons, but not so much after that.
I strongly disagree that color aiding in use points to a larger accessibility problem — perhaps you just mean that if color is necessary for use you've got a problem (which I'd certainly agree with), but while I could get by in my everyday world with monochrome vision, I'm glad I have color cues all around me that aid me in distinguishing amongst objects quickly and with minimal effort.
What do you find so objectively appalling about Metro?