I like the idea of Marcone being an informant for the government; however, I just don't see it. For instance, what does Marcone get out of the deal? He already has little care for the law, so what can the government offer him to make the deal worth it? Also, if he is an informant and gets found out (which he definitely would), Lara, Mab, and the other signatories would not be happy with him. The one thing we know for sure about Marcone is that Marcone looks out for Marcone and, by extension, his people and property. Mab is much scarier than the US government and would destroy Marcone and his people if need be. Although it's a fun thought, I just don't see it coming true.