Sorry, no leads for Scotland.

Taking the hinges off at the pillar (ie. unbolting the hinge from the car) is definately easiest: pull the doorframe U-profile rubber off, remove the B-pillar trim piece, and you have a clear shot at the bolts rather than having to remove the trim from the door and then having to reach inside and work blind. If you're alone this also allows you to keep the door shut while working and opening/lifting it off once all the bolts are out.

Yes, doors are the same between the saloon (sedan) and estate (wagon). Doors did change with the introduction of SIPS and those late doors do not interchange directly with early doors although as with anything Volvo it can probably be made to fit with a bit of work.

Being more familiar with the 940 series I know those have the wiring for the rear power windows in the main harness, so you could just plug in the door, put a 4-way control panel in the driver door and have 3-way power windows! Not sure about the 740, but you might be in luck, I think the conversion kits only came with the door harness. Or change the window mechanism.