Don't worry about having used the online form. Citi accepts it and while it could deny you FCRA protection, it doesn't.
I would try one more phone call to see if you can get somewhere with a supervisor. Try the, "I'm confused" route. "How can I have documentation of something that's a fraud?"
Barring that, CC disputes are just one way to resolve the issue. You aren't entitled to a favorable resolution, just a review and if you are denied, you've got a small claims remedy which will likely result in a quick call from a Citi paralegal.
I doubt that a complaint against Citi helps because it seems as though Citi not only did review your complaint, but reopened it when you asked to have that done.