They are just PVC - no different than the white PVC. You can cut them with any number of saw types. A saw through plastic - hacksaw, jig saw. It should not be an issue to cut them down. I had to cut a number of mine for my project and I used a standard hack saw.
Yes you can use pvc cutters or a chop saw for cleaner cuts.
It's held up fine for a couple summers outdoors. I couldn't say about winter, because it's in storage over the winter, but my impression is that it would probably be fine. Mine's white; it occurs to me that a deep color might fade, but I couldn't speak to that. (I'm using mine for a frame for bird netting over my blueberry bushes.)
My company The Reef Aquarium Shop has used this in numerous projects. Treat it as you would any other pvc project.
Yes - very easy to work with. You will need a union and some pvc cleaner and pvc glue. Cosmetically though you will see the union joint and you will need to be careful with the cleaner / glue to not get it on the exposed surfaces
Yes, it has tremendous strength. We use it to build dog agility equipment too.
I don’t think the pipe is flexible enough to make a hoop. It it fairly rigid.
Yes, you could use this pole as a curtain rod -- and what an imaginative and interesting adaptation of this pipe. It's heavy PVC pipe; the color is solid black; it's strong by itself; and the size allows it to fit over other existing poles or pipes. So, it can provide support to an existing pole, or it can be used as a pipe that provides support just by itself.