Murchison wants to belittle Walter and is condemning Walter for his grandiosity by calling him Prometheus.
This passage is one of several morally ambiguous episodes in the play. Both George and Walter come off as being petty in their bickering and only seem to deepen their codified, class-based differences with no notice being taken of their similarities as men of the same community.
"Flying too close to the sun" is a phrase associated with Prometheus, and it is exactly that action which caused his downfall. It seems George thinks Walter simply wants too much, which is ironic because George has found a way to play the white man's game and now has virtually everything. How dare George make fun of a man who wants to better himself, even though Walter is a bit of a screw-up in the way he goes about things.
I agree with #2, but you also might want to think about George's insult (for what is what it is) in terms of the overall themes of the play. It is clear that George Murchison is rich because he has played the "white man's" game - which is of course something that is distasteful to Bennie. George's insult thus reminds us of the different approaches within the black community there were to their identity and race. George is happy to exist within the status quo, whereas the Younger family are constantly trying to push against this in so many ways. This insult reminds us that the Younger family do not face censure from the white community alone.
I don't think George is actually talking about Walter's connection to alcohol per se. I think he is mocking Walter's "big ideas" about how black men can improve their lot, particularly since Walter's ideas do not include education and white shoes like George's. George seems to be saying that Walter is trying to bring down a metaphorical fire to the "mere mortals" like George. Of course, George is being sarcastic. He hold himself and his education very much above Walter, and finds it ironic that Walter would try to give him advice or disapprove of him.
Prometheus was a Titan god who stole fire from Mt. Olympus and gave it to men. As punishment, Prometheus was tied to a rock. Each day, an eagle would eat away his liver. At night, his liver would grow back so it could be eaten again the next day. The parallels to Walter are lost on him. However, the audience members who know the story realize that George is saying Walter is drunk and destroying his liver. Ironically, Walter wants to bring liquor to others through the liquor store he wants to build with his father's life insurance money. Walter does not understand the allusion and thinks George is making up Prometheus. George is implying that Walter's plans for the future are not promising and the incident also shows that George really doesn't respect Walter. He's letting Walter know that he can't be pushed around but Walter is too uneducated and drunk to realize the full impact of the insult.