I always thought (ok, not always, but since KH2's release) that the bit about him meeting Mickey was just him pretending to be Ansem, in line with the entire idea of it being the "Ansem Reports." Obviously not at first, since Xehanort wasn't a thing at the time of KH1's release. But, it's later easy to explain that inconsistency as just another example of Xehanort "writing as though he were actually Ansem," since he did see Mickey and likely eavesdropped on the rest of that conversation.
Of course, having to use the word 'likely' instantly nullifies this as the true explanation, so it's really just something I choose to believe based on Xehanort's character and there being no other explanation for such an inconsistency at this point.
The fact that there are still so many unanswered questions regarding Xehanort's heartless and the Apprentice and his amnesia and the Guardian and stuff is what keeps me coming back to this series in between games. A lot of series don't have that spark. I just really hope all the questions have a trippy resolution and aren't just blunders.