Once I'm in Java, I am locked into a whole ecosystem and Java work hard to make sure you write OO code. I can't just reach for a lisp halfway through if I think that tool would work better.
Yes, and I've done that before. However, there is still overhead in switching underlying data structures when switching between the two and making sure you don't break the STM among other things. It's hard to share data models between the two.
Actually, you can. The Clojure/Java interop is pretty good (http://clojure.org/java_interop, http://clojure.org/compilation) -- you can call Java from Clojure and Clojure from Java.