(To be specific, I remember someone in the ideological test saying that they self-identify as both Christian and Atheist. And it was 1 person in 13, so that has a non-trivial impact on the results. I don't think that majority of either Christians or Atheists would agree that an opinion like this is a valid representation of their opinions. So how exactly should guessing or not guessing this person's self-description influence the ratings? And should it influence the ratings if the same person would be forced to choose only one of the descriptions?)

Well, that depends on what the test is testing for. If it's about metaphysics, Atheist, if it's about practice, Christian.