Cedric and Bertrand want to see a movie. Bertrand wants to see Muscled Duded Blow Stuff Up. Cedric wants to see Quiet Remembrances: Time as Allegory. There's also Middlebrow Space Fantasy. They are rational but not selfish - they care about the other's happiness as much as their own. What should they see?
The puppy example seems pretty simple. Nonexistent things don't have preferences, so that cell in the table is "n/a". This is the same result as if you just ignored the heisenpuppy's value, but it wouldn't have to be (for instance, 14+2+5 > 10+10, so even if cedric only liked dogs a tiny amount, it'd be a net benefit to bring the dog into existence, but wouldn't be if the dog's happiness were unconsidered).
The rat king is similar to https://plato.stanford.edu/entries/repugnant-conclusion/ , but would be much stronger if you show the marginal decision, without the implication that if rats have a litter, that decision will continue to apply to all rats, regardless of situation. Say, there's 100 rats that are marginally happy because there's just enough food. Should they add 1 to their population?
I think the simulator is too far removed from decisions we actually make. It's not a very good intuition pump because we don't have instinctive answers to question. Alternately, it's just an empirical question - try both and see which one is better.
Your realistic examples are not extrapolate-able from the simple examples. Chickens hinges on weighting, not on existential ethics. There are very few people who claim it's correct because it's best for the chickens (though some will argue that it is better for the chickens, that's not their motivation). There are lots who argue that human pleasure is far more important than chicken suffering.
The parenthood question is murky because of massive externalities, and a fallacy in your premises - the impact on others (even excluding the potential child) is greater than on the parent. Also, nobody's all that utilitarian - in real decisions, people prioritize themselves.
I like your concept that the only "safe" way to use utilitarianism is if you don't include new entities (otherwise you run into trouble). But I feel like they have to be included in some cases. E.g. If I knew that getting a puppy would make me slightly happier, but the puppy would be completely miserable, surely that's the wrong thing to do?
(PS thank you for being willing to play along with the unrealistic setup!)