Yes for duct work. Plumbing I'm not sure in that the screw device seems to be on the light side. I would use something a little more robust for plumbing especially for ferncos or compression patches.
It would attach, but totally inadequate for strength and maintaining a tight fit. Use real exhaust pipe clamps.
Yes... I did. You can put it under the insulation.
The clamp works on any 6 inch duct regardless of the length of the duct.
quantity is 1 package of 2 clamps