Looks quite solid to me. I'd recommend dropping Merciless Combat and Hungering Cold for Acclimation, though. I've enumerated in other posts, I generally find Merciless Combat a little weak for buffing tanking (small boost of threat in the end of a boss fights, trash dies to quickly you won't notice), and Hungering Cold is nice, though it's applications tend to be limited with the amount of aoe that gets thrown around. Acclimation for 3 pts can be very nice in any situation with spell dmg and huge value on fights with constant spell dmg like Sapph.
You could just as easily spend the pts elsewhere as well, like finishing Virulence which is nice for Frost threat for sure.
You're dead on, on all counts above. RS is great single target threat, and your spec should be quite solid on single target threat with better than baseline AoE threat. Also, with 3.1 this build will change significantly. I think Frost is really fairing quite well with the changes though. It's keeping the heaviest survival ability and the new talent positions work well for sharp specs with fun toys.