Assange can be thought of as the source of the email leaks. Assange has been in protective custody for several years, at this point. It wouldn't have been impossible to get in to kill him, but it would have been such an obvious hit that it would have lent credibility and driven interest in the insurance files.
Seth Rich being alive wouldn't exactly surprise me, but that is kind of wishful thinking. There are a lot of weird claims about SR from that night - but no one I know was there or has any further insight than the rumor mill.
The source could also be the server and/or digital intercepts of any discussions about it. It could be one or all of these things. Never put all your eggs in one basket and be willing to run to the finish with any that survive attrition.