1. This only works if you're willing to seclude yourself in the Microsoft Yurt and never leave. Technical benefits aside, history has shown MS is not afraid to abandon projects. It's a dangerous position to take, in my opinion.
And I sincerely doubt a port of Clojure to .NET would result in a 0-difference result. We're back to the exact same problem that plagues other lisp implementations right now, which is to say we haven't really bought much.
2. It's not that different from how things are in the C world, save that you'd share more of your runtime infrastructure. My job has been full of instances where an executable interacts with a diverse set of languages. One example I work on daily includes Erlang, Ruby, Prolog, and C++ all in one space, calling around (in a structured way, of course). The implication that you cannot make modern languages communicate without a single umbrella runtime is demonstrably false.
As a rule, things move pretty fast in Clojure-land. And in the beginning it was dual-platform, so it's not really a new idea either.
This is another reason we should be grateful for Clojure. It will have a .Net implementation soon, too.