I believe ROS phase 2 is 3 minutes until zero mana, but the difference isn't that important. Missing deadens will force your DPS to work that much harder. It's certainly possible, but doing that for a first kill as opposed to having farmed t6 for a few weeks would be an impressive feat. If you reflect every deaden perfectly you will increase raid DPS roughly 25%. It's not a trivial buff.
I've picked up kael in phase 5 after the warrior MT died and that's about as close as I want to get to tanking him
I would like if my guild was a bit more open minded about letting me do bosses like Archimonde (I've tanked the other 4 in hyjal at least once) but a lot of people still have the idea that fear immediately disqualifies a feral tank, which is a shame.
As for crushing blows, a lot of high end bosses don't crush anymore. Azalgor, Archimonde can't. I don't think mother Shaz can either. I may be missing a few others, but it's fairly arbitrary. Illidan can crush, though. I would be interested in seeing what they will do about it with death knights coming into the picture. I was considering making one, but in the end I'm afraid they're going to end up just like feral druids: always second choice.