Well his argument at CT was he couldn't exactly go home since he likely faced punishment for being an officer of Gaius after their failure at the end of 2.0, particularly since Gaius seemed to be acting on his own. I think Nero's primary concern is his own skin at this point.

However, having said that, Nero probably knows the inner workings of Garlean politics a lot better than we do and if he is working with us it could still be with Garlemald's interests at heart in the long term if he fells that the political shift put Garlemald's stability at danger. Being loyal to Garlemald doesn't necessarily mean being loyal to the current Emperor. That could be an in for us to get involved with Garlemald as a nation.