The possibilities are: (as I'm sure you realised already)
1, the regulator (the faults you describe make that sound least likely to me but with electrics/electronics almost anything is possible) you can test that out by connecting 12v to the regulator plug - thereby bypassing all the other links in the chain - if it works properly with independant power then that isn't the cause.
2, the Window Control Unit (black box in the door) you can rule this in or out by swapping it with the one in the other door (they are transferrable in my experience) - if the fault(s) go with the box then you've got your culprit - if not then keep looking.
3, What now remain are the Body ECU - which you have ruled out (but can you really be certain?) and the door switch unit. And of course all the wiring and connectors along the supply circuitry.
My next action (because it's basically free to do) would be to check all the wiring and connectors for damage, moisture, pitting or corrosion - clean and ensure all connectors are tight, dry and clean.
I haven't as yet worked out a way to rule out the door switch or test it - other than to replace it with a known working one (and it has to be a rhd drivers' side from a GT - no others will do the job). So if you have a mate close by with a GT you're in luck if not (and you still haven't solved it) then it looks like taking a punt on a replacement.