1 - Since she has already admitted to distrusting Sara, I think that Yuuki already suspects her of deceit; especially since she confiscated the blood tablets. I'm really looking forward to when the lies that Sara told about Ouri's death 'at Kaname's hand' are truly exposed, and Yuuki can say that she was right to trust Kaname all along
Also, I hope Sara's prison cells will soon be discovered and Yuuki decides to lock Sara up in one just to let her have a taste of her own medicine
2 - Yuuki won't hesitate to use her taming bracelet!! Since its initial introduction in the first arch, it has never seen any use! She'll figure out how to get close enough to Zero to touch her wrist to his neck and WHAMM!!!
He'll go down
3 - OF COURSE YUUKI WON'T KILL KANAME..?!!
I don't think that Yuuki is capable of killing either Kaname OR Zero. So if she was put into a situation where she was pressured to attack him, I don't think that she would be able to go through with it. She might run away, or step down as Dorm Head, or switch to Kaname's side once more, or anything else, really.
4 - Ahhh...I don't know if Yuuki could
I think it would depend on who it was that had become a threat. So far, the only people that Yuuki really cares for are Kaname, Zero, Yori and Chairman Cross. It's possible that she might be faced with the decision of whether or not to take Zero's life if he begins the descent to Level E again. Who knows?
I suspect that there will probably be a death of at least one important character before this manga reaches its conclusion; it's a 'tragedy' after all