How to Proceed: Journal Agreed to Publish, 6 Months Later Reneges
This is a very bizarre situation—I must admit that I've never heard of a paper being rejected because three words are incorrect. It seems that the level of response from the journal is not proportional to the level of the problem. 

I agree with Nate's answer that you will probably need to publish elsewhere. Since the decision has been made by the editor-in-chief, the only appeal would be to the publisher, who would be reluctant to overrule a scientific decision by the editor-in-chief absent signs of foul play.

However, in addition to submitting to a different journal, I would definitely try to get in contact with the editor-in-chief to understand what's going on. If he's someone you already know, I would try to see if I could talk with the person directly on the phone, rather than just email. You might be able to get a better sense of what's going on with this rejection than just in an email. I would not try to force the editor-in-chief to change his mind; I would use this opportunity to figure out what caused the paper to go from essentially accepted to rejected in one step.

If you don't get any response, then you should escalate to the publisher. Such behavior is highly irregular, and should be brought to their attention. (Just state it as: "This is what happened and this is why I think it's unusual." But again don't expect a reversal.)