If they made her fess up, there wouldn't be any reason not to believe her. It's one thing to be evasive, but lying to their faces would not go down well and she knows that. What they wouldn't believe is that the MC has good intentions towards the Empire. They'd suspect him of being an enemy agent whose purpose is to harm them both directly (by crazy stunts like the one with the crystal) and indirectly through manipulated intel (after feeding them some amount of good info, in order to seem legit). All the more reason to grab him.
You are still stuck on the notion circle of "the Top Brass definitely know that Serena is lying to them" point.
Which is odd. I mean Serena didn't outright lie to them to begin with. All she did is hide the source of her info and diverted it to something else more logical.
Why would the Top Brass even waste their time on something hypothetical like "doubt the info source" to begin with, when one of their own claims something else. All they know is that Serena had collected the info and consolidated it and came up with the counter plan to attack the Crystal Beings.
Also, keep in mind, none of the Top Brass are actually at the outpost where the Battle was going on. All info is being shared and decisions being made through online meeting. So, why would they bother with trying to over-analyses something that's happening multiple Star Systems away from them (like millions or even billions of miles away).
Plus, why the heck would the Top Brass even doubt Hiro being some enemy agent, when everything he has done had been helpful for them to begin with?
I mean, he had helped them by,
- kill hundreds & thousands of pirates, helping in managing public order
- eradicating the Federation invading force
- help catch Nobles & Military officials who were acting against the Empire's interest
- emergency transport prototype weapon to an outpost region
- provide critical self-life endangering backup to rescue the collapsing army barricade line at the outpost
- provide critical support to finish of the Mother Crystal
There is also another factor as to why the Top Brass of the military won't doubt Hiro for being an enemy agent.
And that being, Hiro buying and including a Machine Intelligence like Mei in his group.
No enemy agent would ever include an Empire's MI in their group, if they want to be secretive about what they are doing.
Especially considering what was hinted back in Ch.27.
We are talking about a situation in which they already know there's something suspicious going on. You yourself said they already figured out Hiro was behind the incident and even fear others learning of it. Plus, he made it out alive against all reason, as if he long since planned it out. It's already an implausable narrative so if it were me, I'd certainly have that investigated thouroghly.
That was regarding just the usage of Singing Crystal back in Ch.10 plot, not the current Outpost situation.
As for analysis and investigation, they have already done it during the current Outpost plot, through battle data that was collected by their system, and they have deduced that what Hiro had done is basically impossible for any Human to do, even enhanced ones like Nobles. It might only be possibly for MI's to achieve something similar, but as Crystal Beings are also dangerous to them as they are to Humans, even MI's would avoid trying it.
Which is why Hiro received the nickname "Captain Psycho", because they believe only someone who isn't right in their head would do something like that. And someone who isn't right in their head would never be a spy or agent, as its too risky.
You yourself claimed there was a high probability of the intel they'd learn from the MC leaking to enemy nations. That could only happen if the Empire's military was full of spies in high stations. Now you say no one would find out? Make up your mind...
Yeah, things could leak, only if "info is gathered".
If there is no info gathered, how would it leak?
Aka., the military don't investigate or implicate Hiro for using a Singing Crystal, so nothing gets recorded.
The Info from the Invading force is just that a sudden attack from a Crystal Being horde destroyed them, so there is not much info to leak here, as Federation would also know that.
Basically, Hiro's direct involvement won't be known to enemy nation immediately. They might learn about it later, but by then it would be too late to use as political point. So all they could do is target Hiro as their enemy as he had interfered with their plans, so he is a thorn to them.
As for spy stuff. Which country or nation realistically doesn't have foreign spies at some major position?
I mean, there is a specific term as well called "Sleeper Agent", aka., specific people or their family who had integrated themselves in another nation and have no questionable background afters years or decade turn out to be working for enemy nation in secret, just as they strike when needed.
There is also the possibility of military officials or nobles being bribed to do work for them, aka., corruption is everywhere.
I mean, Chris arc already showcased how easy it was for a noble to bribe military officials.
And prior to that, it showed how easy it was for officials to blame someone for someone else's fault and destroy someone's life.
Aka., how Mimi was basically forced to go bankrupt after her parents death, as the official blamed Mimi's parents for the accident which took their life.
Or even Elma, who was unfairly fined with unrealistic deadline and almost got sent to a prison planet, just because the noble in-charge of handling her "damaging military equipment" case, hated mercenaries.
Even during when Hiro was teaching Serena and her Anti-Pirate fleet, about tactics to hunt pirates. He clearly said that there are many normal people in space colonies who help pirates by selling info to them. And these same pirates also trade info with other nations as well to gather tech and materials. There are also cases of illegal goods trading as well, many of which originate from outside the Empire. Aka., its not easy to manage every corner or aspect of an Empire, which is billions of miles in area.
A soldier or policeman might not question a lot of things if their nation is in peace time and it's just another Monday without incident. Try them again when they've already repeled invasions from a hostile country and expect it's agents to be buzzing around. Especially since we're talking about high command, not regular grunts here.
True, but they won't target the one who had helped them solve the issue.
They might investigate that person, and even keep an eye on them in secret, but won't openly interact with them unless needed.
Because if what that person is doing in directly or indirectly beneficial to them, then why even bother and let them do what they are doing while keeping an eye. And just wait to take action until that said person does something which is negative.