If there are some activities I’ve recently started as of 3 months ago but divert from my main passions, should I still list it to show diversity or withhold it and use it for an essay instead?
While you can list 10, that does not mean you should. If the EC’s don’t add anything to your app. then I would leave them off.
It does help to show diversity of interests. Yes, you want a spike, but also some breadth as well as depth. Agree with above, however, only put something on your app that’s interesting or uniquely applies to you. Your whole app should tell a story about who you are and what you hope to accomplish. The ECs just add pieces to the whole puzzle, and give you a chance to list leadership roles.
You can list them, but put them lower in your list. Unless you just joined a bunch of stuff senior year only to beef up your app – that becomes obvious. But a couple new things senior year isn’t that unusual – you are 16-17 years old, still trying new stuff. No problem with that. And don’t worry if you don’t have leadership roles in them (how would you if you just started?)