Let's recap the information we got during this arc so far, whether it's implied or explicit:
1. The most obvious thing first: Kanone is Serie's spy. Very obvious since the first chapter she was introduced. She kept stalling for time so UbeLand can escape and in the latest chapter she DID let them escape.
2. Land has history with the Captain of the special forces. And he's also familiar with the empire's culture and history, either because he was a graduate from the imperial magic school or because he used to live there.
3. Denken's influence and intelligence network runs DEEP. He's trying to act like a feeble oldman when everyone and their grandma know he's not. From the way he described the background of Routine, one of the shadow warriors, we can expect that he most likely already know every shadow warrior the empire has. He even said to have reins on Phrase, the captain of the special forces.
4. The Captain of the special forces is crazy strong that even Macht thought of her as a threat to be eliminated. And that was at least 50 years ago. Either she has become stronger now or she has withered with age.
5. Minus the Great Mage is implied to be the cause of the southern war that killed Fern's parents. Whether it was true or not remain to be seen.
6. The Hero Rasen is now Walross the shadow warrior. He was probably already a shadow warrior when he met the Hero Party 80 years ago, since the empire trained orphans to be shadow warrior not recruited them like normal jobs. So it's probably a case of changing alias and 'role' after his role as a Hero was finished. But in this chapter he didn't mention that he had met Frieren before, so maybe he's actually a double agent or has his own agenda.
7. In this chapter Routine said there are 7 bodyguards of Serie. Frieren, Fern, Stark, Falsch, Sense + most likely Ubel and Land. Even though Ubel and Land had never met up with Frieren and co. in this arc, the shadow warriors already know and count them as Serie's bodyguards.
8. Stark is stronger than some of the shadow warriors, and Fern is being underestimated (like not even a mention) probably because she's masking her mana.
9. Falsch can hide himself from Routine and Iris means he's also specialized in information gathering. Also means, at least Serie's side know of them and the existence of the evacuation route that would be used by the shadow warriors on the day they execute the assassination.
9. A nice callback to the constitution of dwarf in this chapter. Routine said that Walross can handle poison, like Eisen in the chapter where they introduced Sein.
10. On the day of 'Serie Assassination', Denken will no longer be in the empire because he's going to be escorting his father-in-law Glück to Weise. This is a deliberate move by the empire to remove Denken from the equation. But Denken said that 'he will lay down the groundwork' so he's probably has a plan to help Serie without having to be there in person.
11. We still don't know whether the information Ubel and Land got when they infiltrated the party got delivered to Serie or not. Did the special forces take the missive when they caught Ubel and Land? Or did Land's clone deliver the missive? I mean if the special forces got the missive that means the double agent is busted.
12. Land has impressive mana detection skill and Ubel has super human hearing.
I forgot what else so if I miss anything feel free to add