As I see it, we live in an age when tiptoeing no longer cuts it and a spade should be called a spade in no uncertain terms. There were folks in history who tiptoed around the topic in flowery metaphors (e.g. Jesus) or refused to address it altogether (e.g. Buddha) and yet their words were misconstrued nonetheless.

But definitely, one has to see it for themselves -- mere talking the talk won't cut it. But neither will intellectual understanding, not even any particular experience. Still, I can assure you, that once it dawns on anyone, they cannot miss it. Then again, it takes a bit of time to settle into that "aha" that this is really it (then you can even doubt it to your heart's content only to come back to the same).

IIRC, we've already discussed all of that ;) In any case, what's most important is whether your human life has changed (no suffering, free access to inner peace at will, and the like).

As for "ultimate nature", I would not say it's present *in* all humans. There is nothing except "ultimate nature" no matter how any human (or other creature) perceives it (and even that perception is no other).