I thought the reason for the large focus on the Dursts this episode was pretty clear - the obvious implication from everything is they know more than they are saying and instead of helping put him away, they closed ranks (and if the internet is to be believed gave him a huge sum of money to go away). At the very least they weren't helpful. I'm not in love with the ambush schtick wither in this or as a general trope but if as a documentary film maker you get such a gold plated opportunity to visually contrast the pain of the McCormack's with Douglas Durst getting an award for his family values or whatnot, it's sort of negligent not to do it. Just the general optics of all that is worth far more to Jarecki than the actual contact with Douglas. And the rest of the Durst stuff at the home and office goes to Robert's motives. Everyone has been asking, why on earth would you DO this? Well, we got our answer : to really mess things up for his family. Him loitering out side the house taunting them to bustle him away, Bobby Durst in a nutshell.