Setenna is so highly placed in the Alliance command hierarchy that if she's a traitor the ramifications for the Rebellion would be grave. Also, your point about having better means to transmit info is correct.
She was one of the the top two Rebels at the base, and already knew everything that there was to know about Arda, and about other aspects of the Rebellion. If she had/has betrayed the Rebellion you can reasonably guess two things: one, the base could have and would have been attacked long ago, saving the Empire all the grief that operations from Arda has caused over the past few months and, two, no one would have escaped. Plus I need to point out that there is no way that Setenna could have planted those devices herself. She's never been alone (except to sleep or use a refresher) or left the base on foot or in anything smaller than a light transport. It cannot be her.
As I said before, there are people that can be ruled out, easily: Setenna, Doctor Morrick, and Haydon. It would be rather evil to have Bess be the traitor. But if he is, he's not working alone, based on how I've described events in the hangar. As metagame info you know that there is only one traitor. There's an entire, currently hidden, backstory as to why the traitor is a traitor. Whether the PCs ever discover that is optional. IC, the PCs can suspect Qurno or Yalor, but since the alibi each other, neither can be the furtive figure you spotted in the hangar. So you either have one traitor, or at least three. The more traitors there are, the more ridiculous the whole situation becomes. You'd need to go back and ask yourselves if there are that many bad guys, why did the Empire attack now? Occam's Razor pretty much rules out Yalor and Qurno. And General Niall is dead. Now your list of named NPCs is getting rather short, isn't it? Great as far as being in the "audience" is concerned, as the players can guess who it is. But IC there are dozens, perhaps hundreds (the module doesn't say what the population of the base was), of NPCs who it could be.
The PCs need to point the finger at a specific person, with evidence, and all they have is suspicion. You have no evidence that points to anyone. You have devices, but no traces of who planted them. In essence, your investigation is stalled. You need more evidence, and short of fabricating false evidence to accuse someone with, you have nada. The best that you can do is call the attention of the commanders to the problem, and then wait and hope that something (IOW, the story) progresses to an event that might allow you to continue your investigation.