I ask you FalaGringo whether you are a native speaker because this question seems to me very much to be one that a learner, not a native speaker, would ask. The way I'd answer this question to a native speaker would be very different way from how I'd answer a learner.
The general principle (which may strike you as blindingly obvious) is that we say things we need to say and we don't say things we don't need to say.
I don't mean to be rude, FalaGringo, but this strikes me as a bizarre question. Why do you think the speaker doesn't say the exact time period? Do you think there's some kind of grammar rule here? Or is it just that the speaker doesn't need to say the time? Think about the context in which a speaker would say I've already left. (This is why I suggested you write a mini-dialogue—to help you frame the meaning in context.)
Yes, that's a very well formed utterance. (Save the fact that it's actually two sentences—swap the comma for a full stop.)
That's also correct, yes.
In both utterances above, the speaker mentions 9 am because it is absolutely instrumental to the meaning of the utterance. In other words, the speaker mentions the time because it's an important part of the message that he wants to convey. That means that the speaker feels it is necessary for whatever reason for the listener to know the precise time.
The difference between the two utterances is what we call aspect.