My other car is a MINI!!!!
hmmmmmmmmmmmm, some strange Gremlins in there for sure!
ABS is a failsafe system, in theory, when the light comes on, it's totally deactivated and will no longer function. I had similar issues to you, and changed wheel speed sensors and all sorts, before eventually finding it was down to the wiring to the plug on one of the fronts, it had been previously snagged and poorly repaired.
AGU's are super stupid, I don't believe the ABS is linked into the engine ECU in performance terms at all, my ABS light was on for months and I didn't notice, and the ABS light always triggers on the dyno and it doesn't reduce power to my knowledge, so that's another weird one!
As for the speedo, my 98 AGU car with those same wheels was deadly accurate against the GPS, more so than on stock wheels as the slightly larger RR brought the speedo closer to true in fact!
Revs hanging - not sure about that one, I'd say clutch switch on a newer car, but I'm sure AGU stuff doesn't even have a clutch switch. I've certainly never changed mine :S