: instruments with fixed drones tend to build their modes based on a fixed root

Not on the bagpipe. Highland pipes have the drones fixed on A, but tunes can be in A mixolydian, D major, E dorian or B minor (or gapped scales derived from any of those). The interaction between the tonal centre and the drone is a large part of what gives pipe tunes their distinctive character. B minor (or any gapped scale with B as tonal centre) gives a spectacularly abrasive effect, with the tune resolving onto a major-ninth dissonance.