I'll start. 'Borderline' is not often used for what separates one country from another. As a noun, it is used for what separates two classes or categories: The borderline between adverbs and prepositions is not as clear in some cases as writers used to claim it was.
'Boundary' can have a similar meaning, though it also has other meanings. 'Border' is rarely used in this sense, and 'frontier' never, in my opinion.