My and your info both seem to agree that the Amaj chord is not in the Dm key, yet the Amaj sounds good to me and presumably others. Not sure if I understand this.
Actually it kinda sorta it, even though it is buried deep in theory and sounds like a pile of crap. There is a strange, wild chord called a Minor Major Seventh which is composed of the Root, Minor Third, Perfect Fifth, and Major Seventh. In the key of Dm, it would be D, F, A, C#. I don't know where you'd use it (I never would other than maybe a passing tone) but some of the jazz guys may have used one. Play a minor 7th and sharp the top note. It is truly cacophonous, but discordant harmony usually is.