1. "Among countries" justifies the use of "the"
2. Yes, but it makes the first part of the sentence sound like a new phenomemon.
3. Your phrase should be, "Why don't we use...?" Well, using "the" doesn't depend on the noun being countable or not (but yes, "isolation" and "sovereignty" are abstract nouns and therefore uncountable). Look at "the X of Y" pattern.