I've been meaning to write a blog post on my view of what Arc is doing right and wrong, but don't have the time. Instead I'll suggest that the generic language design critique at http://lambda-the-ultimate.org/node/687#comment-18074 is worth applying to Arc; hopefully someone can answer those questions with respect to Arc.
I think all of those questions were pretty thoroughly answered in the original rationale essays for Arc. My only reservation is with question 4, because most of Arc could have been implemented as macros on another Lisp. That might change as it gets fleshed out.