Wow, what a lot of good questions. I should really let someone handle them that has been doing it longer, but there are a couple of them that I know.
I'm also wondering if there is a lot of brutality and pain involved in the lifestyle or if there is a point where a Dom knows to stop before it goes "too far"?
People use what are called safewords. Generally, "green" from the submissive means "I'm good", "yellow" means "not so good, back off or slow down" and "red" means "stop right now". And Doms listen to these and watch for other signs of their subs possibly being pushed too far.

Personally, am a slave, and owned by my Master, who is also my husband. It means different things to different people, I have found. It's generally a more thorough level of commitment to one person than submissive, although not always. Some people just like one term better than the other. Why would I go along with that? Actually, it was mostly my idea. It was just a part of me that I finally admitted to wanting, needing to be submissive, and brought out in the open with my husband.