The first is demonstrably not true judging by the numbers who are highly educated but who do have some belief in God, gods, or spirituality, but as to numbers, who knows. Education doesn't necessarily reflect on thinking skills, and neither does intelligence so often, particularly when one's focus might be in some preferred area rather than being more widespread and perhaps neutral or impartial as to choice. One can only work with the knowledge one has. Motivation and/or personality might play more of a role than education or intelligence as to why some believe and others don't.
As to the second question, plenty can't tell their own inner voice from something external, but they are unlikely to have any mental illness. But, from what I have witnessed on RF, there are some who are more likely to be delusional as to their claims of hearing from God than this actually being so. But then that is just a belief of mine and hence could be equally wrong. I would still not accept such accounts as being factual though - as to coming from God - when there are much more plausible explanations as to why such occurs.