BBM
The case may be closed on Nichole Zotti, but as Rose-Red pointed out, your assertion that she didn't know Peter was in error.
All we have are theories and no one's is "the only rational denouement" until the truth is known. We are generously and politely open to everyone's ideas, even if we disagree.
I don't really see that any of these possibilities you listed can be totally eliminated by using your reasoning/assumptions that I've bolded above. Here's why:
*No, runaways don't always turn up. There are plenty of cold case runaways on Websleuths. Some are dead and and either not found or found and unidentified. There are some who have been able to make new lives under different names and turn up years later. (See my post above)
*I doubt he joined a cult, but not all cults require monetary collateral. Youth and energy that can be exploited to make money are often enough.
*I doubt he was addicted to drugs. But it is possible to die the first time you use them without showing signs of addiction. The body will be hidden by whoever causes the overdose.
*I never deny the possibility of suicide because I've seen too many "unlikely" ones here on WS threads. But I agree that it's not an idea many will embrace. However, we certainly can't assert strongly that suicide is "the only rational denouement."
There are also accidental deaths and the body is never found or found years later.
So many possibilities! While some may be more likely than others, I've been surprised plenty of times on various threads. I've learned never to say "never."
This is just my opinion, but it's as valuable as anyone else's, so let's all start over with that thought in mind.