As you have said, we cannot look into peoples brains, so we have to assume bad intentions, otherwise we would never end up banning anyone, even if they intended to cheat. And again, the rule regarding cheating asks whether an advantage was gained at all, and the rule for forfeiting rounds asks whether the gained advantage was significant enough to have changed the outcome of the match. These are two steps:
1. Was an advantage gained at all, yes or no?
If the answer is yes, the player will be banned and we continue with the second question. If the answer is no, the player will not be banned and we don't even consider the second question.
2. Was the gained advantage significant enough to change the outcome of the match, yes or no?
If the answer is yes, all relevant rounds where the advantage was significant enough will be forfeited. If the answer is no, the result stands but the player is still banned.