I think it was season 4. I believe it's when Ben is taking Hurley to or at least by the cabin, he looks down and we see the ash ring has a break in it. He looks disturbed and they keep going with whatever they were doing.
I don't get why Dogen being in the temple kept Smokie out of it. Plus, if the ash ring keeps him out - how did Dogen's death let him in if the ring wasn't broken? Only thing I can think of is that possibly Jacob made him ageless or something like Richard. Hard to know on that front if Dogen is from the same timeframe as the losties though - doesn't make it impossible though. Perhaps if someone jacob deems the leader still is in the temple, he can't get in. But then again, if that's the case, what's the point of the ash ring?
As far as Kate, Sayeed, and Claire being killers, ergo the dark side - I don't think it's been established that Claire killed anyone prior to meeting up with smokie. Kate's reaction post temple massacre seems to be more WTF is going on as opposed to anything else.