It is said for one to understand art, one has to be part of the culture and the period.
Hi, Im new around here, and just couldnt keep out of this discussion, and actually read the whole thread. anyway, to your question.
leaving aside for a moment the question of what is metal culture, maybe being a part of some sort of a global culture is enough? i believe that it is enough to be a part of the western world, or specifically, aware of monotheism, to fully understand Monotheist by Celtic Frost. I believe it is enough to have something of the popular knowledge / popular memory, to fully understand a band singing about war. and of course, if one has enough life experience (and i believe that in some point every person does), he'll understand a song about death or false love.
The fact that metal is some sort of a close sub-culture is probably due to the fact that most music is plain too harsh for most, or maybe the lyrics are misunderstood, as the stereotype would say Celtic Frost made an anti-christ album, while i think what they have actually written about is the doubts they have in the whole monotheistic subject.
to sum up, yes, i think that you must be a part of a culture to fully understand metal, but the truth is that if you have an internet connection, you probably are a part of it.