i would have to drag my drawings out and check tbh,
the overun shut off valve in a 16i only operates when the throttle is shut and revs are over 1800rpm, on the 16i it gets its +12v from teh ignition ecu whichthe 3i doesnt have so at a guess i would say thats what the speed sensor relay does but dont hold me to that,
is it in a garage with light? if not would have to be a weekend, im about this saturday and could pop round and have a look, sometimes just a fresh pair of eyes is all it needs,
it looks like you have eliminated a lot of problems already so shouldnt take too long,