Hey,
I've had my 81 for a bit over a year, and I think I can answer most of your questions:
Licence plate bolts: I believe 6mm. The spacing is different though, so I only have one holding my plate on. Until I drill another hole in the plate. Its been a year and counting...
Washer fluid resevoir: Yeah, it does headlights and rear window too. One container for everything.
Rear seat strap/hook thingies: When the seats are tumbled forwards, these go to the grab handles at the front of the rear door openings and keep the seats secured. Pretty slick. Also, my rear seats sometimes take a pretty good slam to latch down. Make sure the latches are released first though.
You'll love the 81. Drop me a PM if you're going to be in the Chilliwack area sometime. I'd love to compare notes (and go wheelin')
Jason