I think that was everyone's reaction, at least for those who were relatively unspoiled. Though I have to say, even though I knew in advance what was going to happen, I still was REALLY not prepared for it at all. The fact that it was Kaoru made it even worse somehow; I think the betrayal was at least as hard to stomach as the stabbing.

And I've wondered about that as well. My guess, given that we've now seen that becoming the boss entails more than just heading the family, but also apparently requires literally inheriting the Vongola's sin in some really disturbing way, is that the Ninth suspected that if he waited until Tsuna was older, it would be more difficult to manipulate him into the role. So he had to strike a balance between making sure he was experienced enough to handle it, and making sure he still naive enough to go along with it. Though that last bit clearly fell apart anyway.

HE IS. If they couldn't even kill off his dad in an alternate future, there's no way he's going anywhere. *fingers crossed XD*