"That's interesting. That would imply that it was done deliberately,"
Uh ... As opposed to accidentally?
"rather than as a result of somebody innocently posting a photo of me and then later tagging it with my credentials, (if, indeed, Instagram creates accounts automatically in such cases)."
Where do you find "innocent" anywhere in there? Didn't you say that your your email address was linked to said account?
You own the fscking email address. Email from that address, ask to have the account removed, and suggest (respectfully) that they respond to your email address to verify that you actually don't want that account to exist anymore. Reply to the response. POOF no more account on instagram.
"Most annoying is the fact that I have now received three identical standard form replies from Instagram telling me to email them photo ID."
I seriously doubt the email came from instagram. Gut feeling? Your system has security issues. One wonders what IP addresses the supposed email from instagram came from.
(note: I am not an instagram fan or user)