First, it's tragic and sad that Seth Rich's parents needed to publish an op-ed to discourage "reporters" from calling incessantly for
anything, let alone harassing them over whether they think he may've sent a copy of some emails to someone. I have kids and if one of them was killed on the street, the only emails I'd care about would be those that implicate their killer.
But that's how I read that article -- a plea to stop harassing them, and their official on-the-record statement which, yes, indicates they don't believe he would've been a source, so that people will reference that instead of continuing to harass them.
As for "a mountain of evidence", the only substantive statement as to whether or not Rich actually provided the mails is that investigators apparently told them that they didn't find any evidence of communication with Wikileaks or Assange on any of his computers, but of course, if someone is providing documents to Wikileaks, the fact that they didn't leave evidence of that sitting around on their known workstations isn't really a shocker (nor would it be a shocker if there was evidence on his known workstations and DC police just don't know how to do computer forensics).
I don't think his parents are really equipped to understand whether or not he would've had access to emails -- as most of us here know, most moderately-capable technical people can access virtually anything in a company's network. Remember that Podesta's Twitter was "hacked" during this whole affair because his password was trivially easy to guess; it's not like we're dealing with the NSA here (though note, Snowden was still able to exfiltrate the toppest-secret of the top-secret documents from there).
I'm not really sure what you're looking for, but I guess I can backtrack if it'll make you feel better: it's entirely possible that Seth Rich wasn't the source of the emails and it's probably not prudent to say "Seth Rich provided the emails". I take back the definitive tone and will refrain from such certitude it in the future. The emails definitely could've come from somewhere else, possibly even actual Russian intelligence; ultimately, the source doesn't matter because nobody questions the authenticity of the documents.
That said, it's entirely possible that Rich was a source and that Assange wasn't just heartlessly exploiting a convenient homicide. Personally, I put a lot more trust in Wikileaks and Assange particularly, whose hacker ethos is well-known and lifelong, has a long track record of verified and reliable journalism, and publicly suffered for the courage of his convictions by living in exile than I put in career politicians and media barons who are trying to distract from their ultra-embarrassing performances throughout the 2016 election cycle.
The establishment was so certain that Trump would be crushed that they issued "Madam President" commemorative editions days ahead of the election. We can't pretend like they're not going to try to excuse that with something, no matter how ridiculous they have to get.
No coverup or wide-ranging conspiracy is necessary -- just a handful of powerful people implicitly colluding to conjure a scapegoat out of widespread xenophobia and ignorance, which, I think we'd all agree, is nothing new when it comes to the worlds of politics and mass media.