Well, so far I have determined that, as expected, what sounds typical in this song is in fact unique as hell.
The pre-chorus part tricks you into feeling that it is the same riff over and over again when actually, as you said, it ends different each time. Pretty rad.
For the record, I dont think this tune was played in normal tuning. Odds are it very well could be done in Fripp's "NST" tuning, wherein the guitar is predominantly retuned to 5ths (except the 5th and 6th string which are a minor 3rd apart). This would explain the "drop D" flavored chord movements, and also the extra low notes in the chords themselves, as NST tuning brings the 1st string down 2 whole steps from the original E to make it a C.
I am on a similar arrangement on my knock-off Strat, but I am a full 4th down in tuning on the whole instrument (NST, but 2.5 steps lower on each string). I use this guitar in an experimental/prog drum & bass/guitar duo I am working on with a friend. Anyway, I found some of the riff is definitely in the "lower than standard tuning" registers for sure. Alas, I could not try for long as my daughter (1 yr old) was rather invasive today with here demands to play with daddy.
Tomorrow is another day, another attempt!
Thanks !
NINO