Is a divorce considered to have occurred when the questioner intended to take his wife back, relying on the fatwa of an ignorant person that his wife had been divorced, or is that not counted as a divorce since she was not divorced in the first place?

As long as you have not broken your oath, no expiation is binding upon you, and no divorce has occurred to your wife by intention of revoking it. We advise you to avoid swearing by saying something is 'haram' (unlawful), for the only legitimate oath is by Allah, the Mighty and Majestic, as the Messenger of Allah, peace and blessings be upon him, said: "Whoever swears an oath, let him swear by Allah or keep silent."