This is the tricky issue of dealing with natural zeros that are below the "zero" of happy/meaningful lives (whatever that is).

As I said, this isn't my favourite setup, but I would advocate requiring the natural zero be positive, and not bringing anyone into existence otherwise. That means that I'd have to reject Case_2 - unless there is anyone who would be sufficiently happy about the existence of B with a hugely positive life, that the tiny effort is less than that happiness.

Total utilitarians, your own happiness can make people come into existence even in these non-total ut situations!