On number 3, seeing in black and white isn't all that great. Sometimes I like uncertainty, but I can't have it. It's almost like a cookie that's hung just beyond your reach.
I was not really thinking of a black/white outlook, though I suppose that could be a downside of it. But I think INTJs can be fearless of a conclusion if they consider it well thought through. I know one who was brought up in a religious environment and really committed to this belief far into his teens (making songs about faith etc.). But when new data arrived, i.e. starting to write lovesongs, he found they had the same ability to produce a 'state of grace' as he had experienced in his religious songs and attributed to some godly connection. He was forced to realize that the effect of beauty had to be caused by the strength of music in itself, not by a supernatural communication with a divine entity outside it. He started to look critical on his faith and eventually concluded that it was producing illusions that estranged us from the real powers of truth, beauty and goodness.
Many people eventually quit religion when they grow up. The INTJ version in this case was to become a philosopher and directing his life to advocate humanist ethics, secular politics and scientific freedom. That's being serious. That's going all the way to make the best of your carefully considered new insights. Making them consequential as it were.
Because reaching a point of certainty for a INTx is a somewhat slow process (it seems), I think that most of the time the actual INTx state of mind is UN-certainty. If not about one, concluded, subject, then about the next to need reassessment.