ok here goes.
1) if you want an offspec that eats magic damage blood is not the way to go, unholy is.
2) personially I dont use imp icy tallons but It can be very helpfull. I would say you need to find the balance between threat and the buff to decide if its worth it.
3) 2h frost. No. not worth it, not as effective, not as good as using a 2h in the other specs.
4) to keep rp from overflowing I like to bind rune strike to my oblit and blood strike. other than that frost strike is my main rp skill.
So overall I prefer threat over the buff but in reality its up to you and what your raid needs.