However, it is an opinion rather than a theory ... since it, itself, is not based upon anything other than a failure to believe that crop circles could be formed apart from by humans.
No, on a failure to find any case in which they were caused by anything else. It is one thing to believe that they could have been, but we don't know of any case in which we know that they were, or even have evidence suggesting that they were. "I can imagine an exception to what seems to be a theory" hardly qualifies as an argument, because one can always say that about any theory, it's the epistemological equivalent of background noise. Certainly it doesn't disqualify it from being a theory.