A working definition for skeptics
I think conceptualizing mysticism as a specific form or knowledge is problematic. It seems to me more useful to consider mysticism to be about particular forms of experience.
Experience is always to some extent ineffable, so mysticism wouldn't be unusual in that regard. The particularity of the mystic experience is that it might be very different to other experiences, exceptionally intense, and seemingly disconnected to the external world, potentially creating a big impact in the subject (the "mattering" in your triad).
In my view, a key aspect of knowledge is that it can be transmitted efficiently. This allows us to build upon the knowledge of others, enabling great progress. That's why I think mysticism as a form of knowledge is problematic. It seems that each individual has to rediscover mysticism largely by himself, which would be consistent with mysticism as experience, and not as knowledge.
Of course, some knowledge about specific forms of experience is possible. One experience can be compared to others, one can know about ways that typically lead people to having that experience and so on. But that knowledge is clearly different to the experience itself.