If saliva comes out of the mouth due to moistening the lips, is it permissible to return it and swallow it while fasting, or is that considered excessive scrutiny?

A fasting person is not permitted to intentionally swallow saliva if it has exited to the outer part of the mouth, because that breaks the fast. However, if it exited and returned to the mouth, then was expelled without being swallowed, or was swallowed due to an overwhelming urge, then there is no issue and it does not break the fast. Likewise, moistening the lips with saliva does not invalidate the fast, as long as one does not intentionally swallow it.