I had to edit your question down to one.
The poem is about the speaker contemplating his own death. His “heart aches” and he feels a numbness as if he drank hemlock which is a poison that would kill him. As he’s thinking about death, he hears the nightingale’s song. He is not envious of the fact that the bird is happy, but he feels as if the bird’s happiness is too great for him to imagine; a mystery that he’s not privy to. The speaker also may imply that the nightingale’s happy song mocks him.
That thou, light-winged Dryad of the trees,
In some melodious plot
Of beechen green and shadows numberless, (7-9).
In the second stanza, the speaker wishes for a bottle of wine (vintage) to drunkenly sweep him away into the nightingale’s world.
In the third stanza, the speaker again wishes to fade away with the nightingale. The speaker notes that the nightingale doesn’t have the same cares and anxieties that people have. He (Keats) makes a reference to his brother Tom’s death with the line, “Where youth grows pale, and spectre-thin, and dies;” (26).
The fourth stanza is kind of a turning point. The speaker tells the bird to fly away. He will use the creation of poetry as his escape; as his way to deal with the contemplation of death.
In the fifth stanza, he must imagine the nightingale’s world by relying on the Romantic poet’s best asset: the Imagination.
In the sixth stanza, the speaker says that he has been "half in love with death" because it would be a relief from misery.
The seventh stanza tells of the nightingale’s song throughout history. Clearly, the speaker is drawn to the nightingale for its mysteriousness and its beautiful song but also because it conveys to him a sense of immortality.
In the last stanza, the speaker snaps out of his imaginative revery and wonders how much of his experience with the nightingale’s song was real and how much was imagined. The speaker makes no conclusions about this. This poem is about the speaker using poetry and his imagination to put life and death into perspective. He envies and is enchanted with the nightingale. The nightingale’s song represents a mysterious immortality that he can only dream about. Death may be an escape from suffering in life, but dreaming is an escape from the anxiety of facing death. The speaker definitely conflates his dream with reality for a reason. He wants to make that dream real.