I think there will be a plea deal
True, however, a trial may reveal methods* and create political embarrassment. A severe plea deal is therefore a possibility.
* It's quite possible this news story did not contain the entire truth. For example, it's not out of the realm of possibility the FBI knew before COUNTRY1 turned him in.
He was an amateur spy who was caught using good old police techniques. Whether they knew before shouldn't matter.
COUNTRY1 turned everything over and assisted the FBI in their investigation (signaling the suspect). I'm certain they'll send someone to testify in court if needed.
But it does matter. The prosecution isn't just to get a conviction and enforce the law, it's intended to advance national security.
If embarrassing details come out and harm it, the prosecutors would rather not have a trial.
It's quite possible to imagine various embarrassing details that might come out and are compatible with the story as reported - from possible spying on COUNTRY1's embassy, to how exactly did this engineer smuggle these secret documents to his house.
or the justice dept can accept the plea deal so they can quickly move to other cases.