In that sense being "truly happy" would become a goal. I think such judgements are not "stable" enough to function as an ethical norm. In what way do you think this idea is justified?
I'm not sure its a matter of ethical norms. If we are to say that happiness is the goal of life, and that ethics should direct us to that end, then we haven't presented an ethical norm but the goal of ethics.
On the other hand, there are times when life calls for violence; when hunting for instance. The example I mentioned is an extreme, but I think every person is violent now and again but everybody can still be "trly happy". It is an opinion; a state of mind. Some people are "truly happy" when fighting a war. Those people exist.
How does life call for violence? We make the decision to be violent because violence amuses our desires, and sometimes our short sighted 'needs'.
Sure, some people say they are truly happy when they slaughter others - but bringing up mentally ill individuals who proclaim to be happy is hardly a counter example.
Being content is not opinion. 'Blue is a pretty color' is an opinion.
A priori is not pure reason; metaphysics is. That which exists a priori is the way we think; the possible combinations. That is why people also call maths a priori. I would like to stress the difference between feelings and emotions. That is an important distinction and it makes understanding oneself a lot easier.
A priori is metaphysics, yes, no experience required. But emotion relies on experience. Therefore, emotion is not a priori.
Any distinction between feeling and emotion is purely rhetorical, and can only be justified by context. We might say feeling is sensation, for example. But I do not see the need for bringing up sensation. In any case, sensation is an experience and therefore not a priori. I can't see any other distinction that doesn't equate feeling with emotion.
Math is called a priori because mathematics is said not to require experience - that we can know 1+1=2 without any experience of the sorts of things 1 and 2 might refer to.