Can you clear Knifepoint ridge and get the armor without starting the Daedric quest? Also can I get it even if I'm under level 30? Can I clear Knifepoint ridge with any method or do I have to sneak to get the ebony mail?
Supposedly you can get the quest before level 30, but the main route is after level 30 by reading Boethia's Proving - available in a random encounter - but ensure that you do not cross level 30 threshold whilst in the middle of the quest.
One source indicates that you can get the quest without having read Boethia's Proving (the usual way), by proactively travelling to the Sacellum and sacrificing a mercenary follower. Regardless, the Sacellum will not be populated until the quest has started.
I am unsure about what happens if you kill the Champion outside of the quest. But you certainly do not need to be stealthy, despite the quest info.
However, this quest has numerous bugs associated with it, and I strongly recommend that you create a hardsave beforehand, then proceed as per usual, by reaching lvl 30+ and reading Boethia's Proving, travelling to the shrine etc.