Each case is solved by what society seems to have mutually concluded:
Utilitarianism is certainly correct. You can observe this by watching people make decisions under uncertainty. Preferences aren't merely ordinal.
But yes, doing the math has its own utility cost, so many decisions are better off handled with approximations. This is how you get things like the Allais paradox.
I'm not sure what "moral" means here. The goal of a gene is to copy itself. Ethics isn't about altruism.
Utilitarianism seems to indicate that the greatest good for the most people generally revolves around their feelings. A person feeling happy and confident is a desired state, a person in pain and misery is undesirable.
But what about taking selfish actions that hurt another person's feelings? If I'm in a relationship and breaking up with her would hurt her feelings, does that mean I have a moral obligation to stay with her? If I have an employee who is well-meaning but isn't working out, am I morally allowed to fire him? Or what about at a club? A guy is talking to a woman, and she's ready to go home with him. I could socially tool him and take her home myself, but doing so would cause him greater unhappiness than I would have felt if I'd left them alone.
In a nutshell, does utilitarianism state that I am morally obliged to curb my selfish desires so that other people can be happy?