Mr. Applebee and HamaEstra were conspiring together the entire time. To what end is uncertain, but it likely relates to HamaEstra's plan for F.I.G.H.T.'s finals, the only part besides the opening ceremonies which would have been broadcasted, but were cancelled thanks to the Tournament War. Channel F's position as the only Fuzzy media channel gives Mr. Applebee a monopoly on what events enter global Fuzzy history, and so to recover their plans, he needed to remove the Tournament War from it. His Executive Meddling on the miniseries would make it misleading enough and work to effectively silence any opposing testimony, especially since Fuzzies are pretty dense. When Cid rebelled, however, plan B was he and HamaEstra luring the witnesses of the War and the rebellious staff into one place to take them all out at once, aided by HamaEstras Story knowledge to move the pieces just right.
The monster banging on the door would be Kachooloo, swapped by HamaEstra into a body that possessed power he lacked and sent to handle said witness removal, likely in exchange for Santa's bag. Charlotte and whoever she was swapped into was a test subject for the procedure. Had she found a way into the L.A.R.P., she would likely provide insight that would help stop Kachooloo - though her complete absence from the buildup past the first page means she would probably be more relevant to a theoretical third arc.
But what did Kachooloo swap into? Assuming it's the lump in page 67, it would seem to be part of HamaEstra itself (if very flattened). This wouldn't seem to do the sort of damage it seemingly does to Shambles, but then again Dog was able to kill the hamster in the first place. Given the pitiful state of this first form, an assimilated form made from the remains of other Fuzzies seems the most likely, with HamaEstra's Story contribution making it possible.
How exactly you'd pull that off will be left as an exercise for the reader.
