It's not something I can do myself unfortunately. I'd be happy to try to learn but the car lives half an hour away at a place with no garage facilities, and I don't have enough space at home or a covered working area. All I can do myself are jobs I can guarantee won't result in a non moveable car before the sunlight fades / rain starts.
Since the fluid is low I'll try to find the transmission part/date stamps when I top it up in a week, I'll also test continuity in all selector positions across the NSS pins to see if that's where the problem starts (seems very, very likely). I'll have to order a new NSS and based on the transmission code probably order the type of rooster comb needed, then find somewhere that's worked on 727s before. I guess a new gasket for the transmission cover too - anything else?
Looks like the radio/reverse lamps are on a single fuse if the manual is correct - since the aftermarket cd player is dead (got a NOS tested radio to go back in) and all the speaker wires seem to be cut anyway, that can be my temporary disable of reverse lamps.