A completely obvious check on jury nullification that you've somehow overlooked is jury selection. Juries aren't chosen randomly.

Also, it seems like your argument can be logically extended to be an argument against trial by jury in general, or even against the whole idea of laws that carry punishment. People, especially civilian jurors, aren't predictable or controllable, so there's never any guarantee that some "objective morality" will be carried out by them. And you can't have laws without some person or group of people deciding what those laws are, which almost always will mean that there will be disagreement.