Manfrotto Background Support System (9' Width)

Yes the stand legs are air cushioned.

Yes, I change it by myself all the time by alternating raising each side up about 6-12 inches at a time until I reach desired height. The top bar is adjustable width. You will remove the handheld screw mechanism on one side to slide seemless roll on and then retighten the mechanisms on each side to secure to the stands. It comes with clips to secure cloth backgrounds as needed. I use it for a portable set up and it's awesome. It has a little give at max height.

The width would be based on the crossbar, which would be 108 in this case.

This is already powder coated in black

This backdrop support system is capable of holding up to about 14lbs. I'd recommend using caution if you come across a backdrop heavier than that, but for 8lbs you would be fine with it.

Slightly above minimum height, but below box width. - Minimum height 34.65" (88 cm) - Box Dimensions (LxWxH) 45.5 x 7 x 6.4"

The Manfrotto Background Support System (9' Width) is made of aluminum.

No it is not designed to hold that much weight.