I had a similar situation where, when I first downloaded the Instagram app, I couldn't create an account with the traditional email that I used. Or rather, I could, but Instagram claimed that I had been banned/couldn't log in because I was doing malicious activity or the like. Odd, since this was the first time I tried creating an account.
Eventually, I created a new email, logged in/created an account with the new email, and everything worked fine. (after the fact, a friend recommended that I try uninstalling and reinstalling the app; sounds like it could have worked, but who knows?)
You are definitely right in stating that there isn't a whole lot of information out there on issues like this (hopefully because they don't happen too often?). I remember hearing about an Instagram user who had accumulated around 1000 followers over the years and one day her account was just randomly deleted (or something like that); I'm not sure if she was able to solve the problem. I do remember reading about how some people who had similar problems were able to submit an ID to Facebook/Instagram (the reason being to possibly prove they were old enough to use the app/service?) and then the issue was resolved, but to me, that's giving the company too much information (plus I don't know if it really works or not).
Facebook/Instagram have a phone number you can call, but there's no one on the line (It just tells you to check the FAQ/help page on their website).
Honestly, my best bet would be to reach out to someone who has a larger Instagram following and try to see if you can get them to appeal your case to Facebook/Instagram. I remember reading about how if high profile users had issues such as this, they would be able to fix the problem.
Whatever happens, I hope you can access your account again soon!