Between a rock and a hard place
Needless to say, you are in a very bad, but not hopeless, situation. What you have here is really 2 questions: 1) Take insurance? 2) What to do if the dealer does not have BJ?
On insurance:
If BR2 has any smarts he will insure for at least $10 if you insure. This assures him of victory if the dealer has a BJ (BR2 wins $20 and pushes the dealer with his BJ while you end up with a net push) and forces you to double down if the dealer does not have a BJ.
On the other hand, BR2 need not insure if you don't insure. Again, this assures BR2 of victory if the dealer has a BJ (you loose while BR2 pushes) and again forces you to double down if the dealer does not have a BJ.
So on the question of insurance, I say to take the insurance and hope BR2 does not. You then win if the dealer has a blackjack since you both have a net push.
On doubling down:
If you took insurance, BR2 is now more than a max bet ahead of you and doubling down is therefore your only choice. You can still beat him because his largest lead will be a double max bet less $10 (the $10 is your lead going into this hand). Being forced to double down because you lost a 1/2 max bet on your insurance its not a tragity. With a hard 15, you would probably only take 1 card even if you did not double down. In your situation, probabilities of winning a single or double bet with a hard 15 against a dealer Ace should be very close (don't know the exact percentages).
If you don't mind saying, how did the hand play out?