Very interesting Phil!
If airglow is the major natural contributor to sky brightening, then maybe we need to be looking at the reaction of Earth's magnetic field to the solar winds when planning dark sky viewing! Avoid times of geomagnetic storms etc.
In terms of star light contribution, the 7% must be an average - I'd imagine that there would be significant differences from when, say, the full glory of Sagittarius is arching overhead, to when the MW is lipping around the horizon.
Something to think about in the quest for darkness, great stuff!!
Cheers -