First of all kudos to you for trying to find answers here. If you need to know about DL's the information is known by someone. In regards to your first question unless it's a very loose BM a diaper will hold it in. I've personally never had one leak "matter" onto another surface. In addition to the diaper though I also wear plastic panties as Bettypooh stated earlier. Myself, as an experienced diaper wearer, get a feel for when a diaper needs changed. Wet diapers are a little more tricky. If you wear one long enough it will leak but that's where getting a feel for it comes in. Wearing the same diaper brand all of the time helps you to know when it's going to be the right time to change it. You can't wear a Depend anywhere near as long as you can an Abena. If you are really concerned about possible leaks you could ask him to use a plastic backed underpad when he sits on the furniture. They're sold at most drug stores.
The answer to your second question is anybodies guess. I think my DL tendencies come from being super close to my mother, ie, a momma's boy. While others can be traced to being diapered past potty training age and their parents made them wear a diaper to try and humiliate them into using the toilet. I hope you can make some sense out of this and that it helps you deal with what you are going through.