Dammit. I REALLY like this theory
. Very very sad because Bayard is one of my favorite characters. Which is, of course, a perfect reason for him to be the traitor.
I agree with the earlier post about Abigail being mentioned more than was necessary being of interest, maybe she is the traitor.
The one thing I can think of to counter any of it is that both the Spirearch and Grimm mention that they think the traitor is in the Guard. Grimm even specifically says that as being one of the reasons Addison used the criminal guilds of Landing to be ready for a potential attack, and did not dispatch additional members of the Guard. Of course they could be wrong, and an aristocrat or high ranking member of the fleet would certainly be able to find ways of their own to gather such information. But for the moment they are proceeding from the assumption that the spy is in the Guard, specifically.
(oh hey, what about the Spirearch's batman?)