the unknowableness comes from the difficulties of causally explaining the phenomena of subjective experience in terms of physical processes. we can explain information processing, in those terms, we can explain automatic unconscious biological behavior in those terms, but we have yet to develop an understanding of a causal (as opposed to merely correlational) relationship between physical phenomena and subjective experience. obviously the fact that brain damage exists implies that, whatever the nature of consciousness and subjectivity, it is affected when certain physical structures are interacted with.

I'm not entirely sure what you mean by absoluteness vs. contingency in this case, but as far as philosophical implications, what the existence of subjective experience MAY imply, is different kinds of metaphysics or ontologies or epistemologies. A physical realist worldview is that ONLY physical matter and physical processes exist and are knowable, and that subjective experience is "illusory" or not real or unkowable. What the existence of Qualia implies, is that there is at least a subjective, and perhaps even semiotic, component to reality, in some fashion, that we have access to in some capacity. Some philosophers use this to argue for Idealism, which posits that ONLY 'mind' or subjective experience really 'exists', and that matter is 'illusory' or 'not real'. I find both of these extremes unconvincing, I personally like Analytic Idealism myself, which is an ontology that posits that physical processes are the extrinsic appearance of mental processes, and that mental processes are the intrinsic appearance of physical processes. Other interpretations may be that there is some hidden, more fundamental reality from which both the mental and physical originate.