Yes, screws are included.
Depends on what legs you use, ho long the screws into the wood are and whether you have them straight down or at an angle. If you have them straight down, they can hold whatever the wood can support; at an angle I would not exceed 30 lbs.
Yes. Each bracket has a straight and angled mounting socket. Very convenient.
Yes, I've used them on couches, coffee tables. Very sturdy.
Although I haven't purchased them yet (they're out of stock), I believe you can screw in the legs either straight up and down or at an angle. I inadvertently bought the other plate which only allows for straight installation. Hope this helps a little.
Yes. All the threads match.