given that his decisions revolving around Harry, James, and Lily Potter were all purely emotional

And they weren't terribly good decisions, were they? IMHO, that supports his being a T rather than an F, because he makes more mistakes when he's being emotional.
His devotion to Lily doesn't require him to be an F - INTJs can be extremely devoted and loyal, it's just that they hold most people at arms length, but once someone gets inside their shell...

Mr. Darcy

Really? He's certainly an introvert, but I hadn't pegged him as an INTJ.