As I said, this was extended to 15 years, so it recently expired, not 5 years ago.
No, self-deployment isn't a concern. MB is/was concerned about how well the bag material itself holds up. Of course there is no feasable way to "unpack" the bag material for inspection, but experience has shown the bags to be good out to 15 years.
Your first line of protection is the seatbelt. I'd hope for the best with the original airbag, a new one if I remember another poster's question (do a search) is somewhere in the neighborhood of $1500.00.