Just to clarify, Brian, was the home-key drone the same on push and pull?

in the key of C, you would be hearing G, D and C notes together, making an edgy, clashing sound (can Phil tell us what that's called?).

Thumbing through my I Spy Book of Chords... G/C/D is a Gsus4, which usually sounds 'edgy' - it's just asking to resolve back to G/B/D*. If he was playing the full G chord, OTOH, you'd have G/B/C/D, which is dissonant - although you could also think of it as an inverted C maj 9 (C/E/G/B/D) with the E missing...

he's accompanying a F mixolydian melody in the key of Bb, so none of the chords are the ones you'd normally expect

Possibly because he didn't realise it was Fmix? It sounds as if there were a lot of happy accidents in PB's arrangements, and maybe this was another.

*Not always. The refrain of Radiohead's No Surprises modulates from an A to an Asus4, with the kind of 'release' effect that you usually get from going the other way. Mind you, it then resolves onto the home key of D, so you could say that it's actually going from A to an inverted Dsus2 & from there back to D.