- Who owns SE? Is it an individual, an organization, or some international government initiative?
Stack Exchange has a CEO named Joel Spolsky. It is an incorporation. I don't know anything about their division of shares.
- What amount of power does the owner have? (For example, could the owner just delete the sites one day and start selling the content?)
I suspect it would be a pretty big deal to delete the sites, and they probably don't have a big red button to do it> Stack Exchange is one of the most popular sites in the world, so there's enough redundancy that I can't imagine it'd be an easy task. Someone would get stopped before doing any real harm. It's the same as Amazon, Facebook, or any other huge company. You can read more about that and the licensing on legal.
- Do users/moderators have any say in the basic functioning and policies of SE? If yes, can a majority of users make any changes in the basic principles of how the content is owned and dealt with.
Users and moderators have a huge amount of say, which you can review in status-completed posts. But at the end of the day, it is Stack Exchange employees who make real changes, and there are some fundamentals (like licensing) that I don't imagine will ever go away, even if they legally could, which I don't imagine they can (I haven't actually read the license recently, but generally once something's licensed out there, it's hard to take back).
- Why doesn't SE merge with other popular sites such as WikiAnswers, Yahoo Answers, Stack Overflow, etc?
Stack Exchange owns Stack Overflow, and wants no part in WikiAnswers or Yahoo! Answers. Those sites serve very different purposes, and SE has more than enough of the market share to be happy letting them coexist (I imagine).
- It's okay if moderators have greater power, but shouldn't ordinary users be able to flag any kind of task so that a moderator could look into it? (For example, if an off-hold question has been edited to make it appropriate, nothing can be done until a moderator somehow happens to find it. A normal user can not flag, vote or do anything to bring it to the moderator's attention)
Well, yes. Users can vote to close questions, flag posts, up- or down-vote posts, and everything else you've listed. You can read about how much involvement moderators really have here.
As for on-hold questions, users with the privilege can also cast reopen votes, and any edited post will end up in a review queue. Moderators very frequently have no part in that step of the process.
But this all said, I should make it clear: nobody is out to get us users. It's in the best interest of Stack Exchange Inc., Joel, any developers working there, moderators on any site, and mostly users on any site, to keep things running smoothly and keep the majority as happy as they can be. SE has a good thing and no interest in ruining that. Selling all of our content, even if they could do that, wouldn't help anyone, and neither would deleting sites.
But that all said, your content is licensed under a Creative Commons license. So you shouldn't post anything that you aren't comfortable with other sites "taking" as well. And just like services such as Facebook, it's probably bad practice to host very important documents there (not that I imagine you even could on here), since yes, servers can go down and such. But you really shouldn't be too concerned, day-to-day. A lot of people would be very sad if all our content got destroyed, so again, it's in everyone's best interest to not let that happen.