My Variant of Utilitarian Ethics

Here is my basic formulation of utilitarian ethics. Below, I will raise crucial issues regarding certain problems with it.

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When determining the ethicality of decisions, what's pertinent are a person's intentions, the net effect of his decisions on societal happiness, and the difference between his intended and the actual effects. 

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- Should harming oneself and thereby effecting a net loss in societal happiness be ethically justifiable? Why, or why not?

- Does this formulation assert society's primacy over the individual? If not, why might one think it does? If so, is that necessarily bad? Why might it be good?

- Is there any plausible, realistic, hypothetical situation where one's loss of happiness and a resultant net increase in societal happiness is seemingly unjustified? If so, then under what code of ethics would this be determined as unjustified?

- Perhaps it should be added that people's level of happiness should be considered (e.g. harming a person with 4 units of happiness is less justifiable than harming one with 5, particularly if and to the degree that one is happier than that). Maybe this addition should only apply to malicious decisions, as well as recklessly negligent ones.

- Applying the above addition, how should malicious or recklessly negligent decisions be judged if the perpetrator is less happy than the victim? Should we not consider their respective happiness? If so, why not?

- Is there a such thing as a "moral duty" or "ethical imperative?" If so, at what point do decisions become a duty? Is there a meaningful distinction (or even any at all) between living an ethically justifiable life and obeying a moral imperative?

- Since not all justifiable decisions (e.g. kicking a rock, staring at the ceiling, eating a fruit, etc.) would be called "good," "morally right," or the "right thing to do" by most, could we conclude that there's no such thing as a "good" act? Or, if only those decisions that produce a significant net increase in societal happiness can be called "good," how should the line between insignificant and significant increases be drawn? Are there any possible decisions that lead to a significant increase that most would not consider "good?"