If you take the literal meaning of borderline, it is to be on the border of, or at the edge of, whatever the noun or verb is. So for example, the borderline to a country is the edge of the country - neither in nor out of it, but right on the line. To be "borderline" anything suggests being "on the edge of" or "close to" or "almost" at the stage of the verb or noun it is describing.
So in this example, to be "borderline boring", means being almost boring, or, "on the edge of" being boring. Therefore, your first answer (number 1) is the most accurate. I hope this helps.