business auto policy, excess follow-form, adversary proceeding on policy interpretation, self-insured retention, SIR, actual and full payment of SIR, coverage conditioned on SIR, choice of law, interpretation of unambiguous contract, standard for subject matter jurisdiction, standard for motion to dismiss, FRCP 12(b)(1), FRCP 12(b)(6), justiciability, standard for joinder, FRCP 19, maintenance clause