Isn't "cześć" more of a "hi"? How official is it? Can I use it to greet a super-important person in official situation?
Definitely not. I always thought that "hello" is only informal as well, but English natives on Duolingo make me change my mind about that a bit. Anyway, you can use "cześć" with your acquaintances, colleagues and friends, your family (at least close family), and probably strangers that seem to be about the same age as you or younger, when the situation seems informal enough (school/uni/concert/club/bar etc.). So yeah, more of a "hi".
Witam is "welcome" ( I welcome). It is now often used as the greeting at the beginning of e-mail, but this is complicated situation - it's less informal than cześć, but not formal.
Using when having real life conversation - I don't know people who use it this way. Using it when you are not a host would be weird. Also "Dzień dobry/Dobry wieczór" are much better.