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<blockquote>why it is acceptable to reduce what appears to be a fairly complex capacitance “network” down to a single value for use in the “allowable capacitance” calculation for a barrier?</blockquote>

Because you only care about the worst case when it comes to Intrinsic Safety.  In this usage, there is an upper limit on allowable capacitance to limit the amount of stored energy.  It doesn't matter in which specific distributed capacitors this energy is stored.  For the worst case, you have to assume all the individual capacitances act together as a single larger capacitor.

This would be a pain to calculate, but fortunately the cable manufacturer has already done this for you.  For any one wire in the cable, you want to know the capacitance to everything else if it were all tied together.  That's exactly what your "C-S" spec is.  Use that for IS purposes.