Ok. I'll try to answer to best of my abilities.
1) As far as people know, 4 or 5 centuries but in secret. However in the last century, there has been a mass awakening in people being discovered with superpowers.
2) By beating the s&!t out of them whilst spouting a corny catch phrase before throwing them in a woefully under equipped prison.
3) The same way they react to "regular" villains. Sending cops to arrest them. Then SWAT (or their equivalent) if the cops fail. Then finally the military.
4) Yes, but not any who have made contact with the human race. Maby the odd Superman esque intergalactic refugee (but not a OP as him).
6/7) Possibly, though experiments and research are going on. No progress has been made. Though I wouldn't be opposed to someone who wants to play a villain who has found himself in "our" dimension in some accident.
8) Because the Heroes formed the "Invincible Guardians " to try to pool their resources to combat the ever growing threat posed by the villains.