On the object level, I think you are almost completely wrong.

You say, "There is not one culpable atom in the universe." This is true, but your implied conclusion, that there are no culpable persons in the universe, is false. Likewise, there may not be any agenty dust in the universe. But if your implied conclusion is that there are no agents in the universe, then your conclusion is false.

But if there are agents in the universe, and there are, then there can be good and bad agents there, just as there are good and bad apples in the universe.

Richard Chappell, I think, has used Singer's own argument against him. Suppose you are jogging somewhere in order to make a donation to a foreign charity. The number of expected lives saved from your donation is 3. On the way, you witness a young child drowning in a river. You have a choice: continue on, expecting to save 2 lives overall. Or save the child, expecting to lose 2 lives overall.

Everyone knows that the right choice here is to save the child, and that the utilitarian choice is wrong.

The utilitarian error is this: it is asking, "what actions will have the most beneficial effects?" But that is the wrong question. The right question is, "What is the right thing to do?"

(Edit: there is another inconsistency in your way of thinking. If you assume there is no culpability in the universe because atoms are not culpable, neither is it worthwhile to save human lives, because there are no atoms in the universe that are worth bothering about.)