Flip lever clamp seems good.
For what , I don't know.
Umbrella , flag pole , pipe to transfer something.
i checked irrigation pipe and didn't find something similar. Could find a use somewhere though like on a fill drop pipe or something that needs adjusting.
Arc - I am guessing at the size based on the bricks in the background. Not completely reliable but all I have to go on.
As for it being open on one end, as others have said, the OP mentioned it in the first post. If you look carefully at the first picture it's pretty obvious that one end of the cylinder is capped off. You can see the weld seam. That's why I don't think it's part of an umbrella.