1) Is it generally true that when you borrow a chord from another mode you are substituting a chord (from the new mode) with the same root or position as you would otherwise play if you were using the diatonic chord? In other words, a ii for a II or a iv for a IV. Or is that not a very helpful way of thinking about the possible chords at your disposal (e.g., if you were trying to compose a progression).

2) My understanding is that borrowed chords are fleeting. If you borrow more than a single chord, it starts to be more appropriately thought of as a key change. If that's the case, why say that you are borrowing from any particular mode? Why not just say that you tare playing a non-diatonic chord that for whatever reasons works well in a particular spot. For example, if you use a bVII instead of a vii-dim when playing in a major (Ionian) scale, you can say that you are borrowing it from the Dorian mode, but you could also say that you are borrowing it from the Mixolydian mode or the natural minor mode (i.e., since they all have VII chords) or, I would think, that you are not borrowing it at all but just playing an out of key chord?