Yeah. I caught the Montjoy Yucatan, Chichen Itza connection before. I didn't know all of what to make of it. But I think it likely he was scouting out the Reds.
I didn't think much to include him in the Grey Council but now that you mention it, it sounds very reasonable. That said, I sorta wondered what Peabody or whoever did with that tidbit of knowledge that Montjoy was scouting out the Reds. I, paranoid me, was thinking that someone would conspire against him, or try to turn him. Who knows though.
It could be that when Martha Liberty gave that information, Nemesis, Peabody, Black Council, Reds or whoever acted against him in some way to turn him.