- or between other administrative divisions ('I live near the border between Devon and Cornwall')?
Yes, of course. There are nearly as many 'meanings' of 'border' as there are contexts. I was answering the question with reference to the line between countries.
With 'boundary' isn't the focus more on the line that divides 2 areas whereas 'border' is a bit vaguer ('They live in the border regions of northern California')?
That's an adjectival usage. I agree with your example, but don't see its relevance in the context of the question.
Having said that, we wouldn't say 'boundary' in reference to countries - even the line is the 'border line'.
Maybe it is where you come from. I, too, would call the boundary between England and Wales or England and Scotland a border, but between say France and Italy there's a frontier.
As for 'frontier' , I don't think we say that so often in reference to countries. In most cases, we would say border - so 'the France-Italy border' not 'the France-Italy frontier' - wouldn't we?