Everyone deserves dignity. Yes, everyone. Mark does these interviews with the pure intention of the interviewee getting an open unregulated platform to say what they want; to tell their story or not tell their story, to express their anger/frustration/hurt etc...to express their gratitude(or lack there of), understanding (or lack there of) and anything and everything in between.
He doesn't have to use disrespective or "jaw-dropping" methods to bait individuals to come out with the "dark stuff" because their lives are dark all the time. For many this interview makes them feel human. A long forgotten feeling of being seen and heard. He uses generic questions in a light non-aggressive manner to help them feel comfortable enough to start talking. The questions he asks are quite crucial in understanding how one's early years can set one up for future or success.
Of course everyone makes their own decisions,will power bla bla bla. But in a world of so many people coming and going from this life daily you can only rely on statistics to get an overall macro idea of how humanity works and it is evident with each and every interview that having two parents (or even one) who love and care for you the way parents should will almost always give a better result than having 1 parent who abused and starved you or jumping through foster homes your whole life. That being said, it is very impressive and noteworthy and inspirational to watch the interviews of those few individuals who had the worst childhood imaginable and some how found it inside themselves to break through the statistics and immerge on the other side as a well-rounded, kind and empathetic individual focused on helping others break the cycle. Someone somewhere down the line treated them with dignity and respect and perhaps assisted with resources or perhaps just made them see their self worth and it made an impact on them and they changed their lives. Kindness and empathy are verbs of action not words of passivity.
Your question about how they "clean up in between clients" tells a whole lot about you..I won't go into full psycho-analysis but just know everyone can get a lot of benefit out of talk-therapy and if you are lucky and persistent enough to keep trying until you find a great therapist who you click with that understands your specific struggles and how to connect with you to get results (if you are willing to do the introspective work), you can really grow as a person.