Worf murdered an unarmed Weyoun,
Weyoun had it coming. If you don't want to get attacked by a Klingon, don't capture him, don't torture him, don't threaten him with execution, and don't insult his "girlfriend" in front of her. And you could in fact argue that Worf was defending Ezri's honour by killing Weyoun. Under these conditions it's fair game. Weyoun was the one challenging Worf. I don't see anything dishonourable here.
I agree. I think the Klingons recognize that what Weyoun did was enough to provoke an attack (any of the things he did), and thus his being unarmed when the attack came was his fault and not Worf's responsibility.
And even if he broke some rule by challenging Gowron... there's a difference between following the rules and being honourable. Some of the most dishonourable Klingons were always following the "rules". I guess Gowron wasn't breaking any rules when he dishonoured Worf and his family again in "The Way of the Warrior". But it still wasn't a very honourable thing to do.
It does not appear that Worf broke any rules there. Although it was more Martok's place to do it than Worf's, Klingon society seems to believe that if the leader is a traitor or a coward, ANY subordinate has the right to challenge him.
For my money, though, Worf misunderstood the quote from Kahless: "Great men do not seek power, they have power thrust upon them". That means that, even though Worf did not do what he did because he wanted to be Chancellor, nevertheless the power is his. Because he was the one willing to put his life on the line to stop what he knew was wrong.
And it builds so beautifully with Ezri's earlier comment about "Who was the last Chancellor you respected?"
They should have made him keep it.