Summary: the problem with Pascal's Mugging arguments is that, intuitively, some probabilities are just too small to care about. There might be a principled reason for ignoring some probabilities, namely that they violate an implicit assumption behind expected utility theory. This suggests a possible approach for formally defining a "probability small enough to ignore", though there's still a bit of arbitrariness in it.
Which particular event has P = 10^-21? It seems like part of the pascal's mugging problem is a type error: We have a utility function U(W) over physical worlds but we're trying to calculate expected utility over strings of English words instead.
Pascal's Mugging is a constructive proof that trying to maximize expected utility over logically possible worlds doesn't work in any particular world, at least with the theories we've got now. Anything that doesn't solve reflective reasoning under probabilistic uncertainty won't help against Muggings promising things from other possible worlds unless we just ignore the other worlds.