I am not convinced by your argument that we are very unlikely to be Boltzmann brains or in an Everett universe. For instance, in an Everett universe, indeed multiverse addresses are very long, which means that your probability of being at a given multiverse address is very small; but that's counterbalanced by the fact that there are many instances of you in identical-looking situations at different addresses. (And what we observe is not the event as you've defined it but only what we are able to observe of the event.)