Regarding the six CR123a's and whether that's overkill. Well, for one thing, six CR123a's take up very little space in a vehicle kit, so that's not good a reason to pare them down. If solely for use with the X5, then that's a good long time and I can see you might think it's overkill. But here's another angle. Your B42 can only run on its highest setting for what? Like 20 minutes? If you need bright light for more than 20 minutes, then your only option is to keep putting fresh cells into the B42, so from that perspective, six extra cells isn't overkill.

I don't see any mention of extra batteries for your headlight.