In your space you have the most aggressive blackhat seo actors (supplements) looking at you as the bad guy for exposing them, I wouldn't be surprised if you discover a ton of bad backlinks from domains that have a terrible reputation.
Source: lost over 100,000 unique Google visits per day because of a malicious competitor putting unsolicited backlinks to our domain without our knowing and it took months before it came back.
> lost over 100,000
Sucks. You'd think G would be smart enough to ignore, not penalize.
Negative SEO shit has been going on for over a decade now...
Let me know the progress, I'm fairly certain this will help you out big time, unfortunately it's a bit of legwork and it takes time to recover.
Even if somebody inside Google wanted to fix your problem they're not able to respond on search ranking algorithms or exceptions. I heard from a Google search engineer that other than a curated blacklist (with many controls in place) they have removed the ability to influence the rankings (positively or negatively) with human intervention. This is due to "plausible deniability" over them playing god with search rankings.
Maybe someone with some authority will see this and review a penalty - albeit without responding to you - but usually this will come back in a few months if the underlying cause isn't resolved.
Best of luck.