A key is a thing you use for locking and unlocking a door. Or a thing you sing in. Try not to overuse the word as an adjective meaning “important”. Consider writing “important” or “major” instead, or see whether you can cut it out altogether.
Hyphenated as a noun or adjective (as in a kick-off meeting). But two separate words as a verb (the project will kick off tomorrow).
Kudos is a word of Greek origin originally meaning “praise and honour received for an achievement” and now also used in a looser sense to mean compliments or congratulations. It’s singular. You can only have kudos. Although some dictionaries recognise it as a valid back-formation, you can’t really have one kudo.