
I might be wrong in getting your question but from what I understood through it here's what I think.
They were one of the key family with the armed power in the empire and the emperor/king might have always felt some sort of threat that they might betray or not. So when the trap was laid and blonde's family got caught and was slaughtered for treason was probably also a way for the previous emperor to eliminate one of the threat (if he ever felt so). As for the letter why the family didn't show to prev emperor might be because if they carried it around Leroys probably would have destroyed. And if I'm not wrong, the blonde's family wasn't even given the chance to prove their innocence. I might be wrong duh (⊙…⊙ )

Yup they were executed directly emperor army came after he saw the proof. And killed everybody. Keena's father sent her away with the proof when he understood the things will be deathly for all of them, if i remember well she was 12th and her talent wasn't unleashed yet, it was out on the day of the death of all her family....
Not to be that person (especially as I always bang on about suspension of disbelief) why didn't House Erebus (Kianna's house [blonde lady]) just speak to the previous Emperor??
I get the need for secrecy, but surely they would've spoken to at least one of the previous Emperor's key aides or trusted ministers...? ╮( ̄▽ ̄)╭
It just seems a lot to risk, just for a letter that *claims* to be from the emperor...
Especially, when getting it wrong could (and did) lead to the erasure of the entire household?? ( ̄∇ ̄")