Hmm, I might want to take a stab at some of these questions...
- Is "deep" music and playing, a niche market and by its very nature destined to not develop international followings? Because, it seems like it is. I am generally curious if a person were to seriously search the depths of their soul throughout their lives, and on top of that, be able to express it and communicate that in music, is it likely that it would actually be far-reaching, or is it somewhat the reverse? Does expressing oneself deeply become something that only certain people end up resonating with and/or at certain times within a life vs. masses of people?
Well. On the other hand, I can't think of a very sure way to justify my answer. But maybe I'll just say what I think

with the caveat that maybe it is just what I want to believe.
It sounds like what you're asking is whether our deepest truest feelings are completely unique, so that if we could really express them, no one else would "get" it--or at least, no one else without similar life experiences. Do those experiences define who we are to such an extent that maybe some of us can never understand each other?
I tend to believe that, while the actual things different individuals experience in life are certainly different from the outside, a human is a human, and we all experience the same range of
inner emotion. I, for example, have never experienced genocide first-hand, but I was really sad when I lost my grandfather--actually that might be a bad example because my grandfather did have a very long and almost totally healthy life, so there wasn't a sense of anger or unfairness or frustration. But, some friends of mine lost a very close friend of theirs just shy of her 20th birthday, and they were very angry and heartbroken, maybe more than they ever had been in their lives. Is their sadness less than that of a victim of worse misfortune--like genocide? I guess I picked sort of a touchy subject and it's difficult to be presumptuous enough to say yes, their first-world sadness is equivalent, because objectively one of those events is clearly worse than the other.
I guess my point is that all of us, at some point, have probably felt despair beyond anything we think we've felt before. However low we may be objectively, it
feels like rock bottom. So there are commonalities in our subjective experience, even though our objective experiences might be different
Oh, but wait... we were talking about music

I think one of the great things about music (and I guess art in general) is its ability to speak to subjective experience in a pretty deep way--deep enough to get to the bottom of ourselves, where, despite surface differences, we're all human with human thoughts and human feelings about our lives. That sounds like a public service ad for arts education or something, but it's actually something I really believe...
I am also curious if this is a quality that people are actually looking for in music, or is it strictly something else that seems more appealing? (Like, "entertainment" though I'm not suggesting entertainment can't be deep). If there are works or particular performances of works which you feel might fit in this category, whatever works/artists those are, I'd be very interested to know if you'd be willing to share! I'd also be interested in any other thoughts regarding this that you'd like to share.
This I'm not sure about... I would definitely say that the top musical experiences of my life have truly moved me, in some way that seems like it should be called something more than "entertainment", but I think there I'm just getting into a debate of semantics; there's no good reason "entertaining" couldn't be used to describe an experience like that. Maybe it was just a really high degree of entertainment...
(I'm rambling because I'm procrastinating. Sorry >_<)