Part #13 for the 2005 is the big rubber boot but the diagram doesn't indicate the small bearing located inside at the top of the boot, same for the sort of large C clip that secures the boot to the floor

From VIDA (their bold fonts ): Filling and checking the oil in the Active on Demand Coupling: Use correct oil Volume approx. 650 ml. Fill oil up to the filler plug. Suck out 0.1 l oil. Caution! If the AOC (Active on Demand Coupling) has been replaced with a new one, the car must be driven carefully ...

Added part number to title for search

The rear differential grease is API GL-5 / SAE 80 W. Volvo part number 1161620.
The API GL-5 / SAE 75W-90 for the angle gear will do just fine. Volvo part number 31259380 or equivalent.

1) If you can get 80w-90, it will be a little more robust than 75w-90. However, the 75w-90 will work. It's the same concept as motor oil - get the narrowest viscosity spread that suits your climate/application. Apples-to-apples, an 80w-90 will be more robust and last a bit longer in the same way a 1...

Oro has answered the main questions you ask. Only thing I would add is that neither the angle gear and rear diff use grease. Both use an oil. I am not sure what it was as I recall at the time I did this about 8 years ago, they were a special mix sold by Volvo but I think I recall someone identifying...

What is the construction of the rear drive differential? Is it a limited slip differential? I don't have enough experience looking at the printout to know the difference. The part I marked from the diagram above in red is a Pos-Trac LSD differential (that is what caught my eye). The parts I marked ...

As far as I know it's just a weighted vibration damper. No maintenance needed.

In my case I wanted to change all 200k miles worth of fluid in one session rather than drain 2 or 3 quarts every 100 miles or so. I would rather spend a couple hours now standing upright vs putting the car on ramps every weekend until I'm confident all the fluid is changed. Also I feel I get a more ...

The answer is in the name. It dampens oscillations. Does this by absorbing some of of the energy of a vibration.
It is a passive device, no maintenance required.