So you think they should have slowed the plot down to get us more attached to Mimi before killing her off?
I don't see how the Youmafication serum not having been setup before use is really an issue given a massive part of the series are the mysteries of Youmafication. These guys clearly know stuff our heroes don't, which is why they were after the book. Them having a serum like that just further establishes they hold keys to Youmafication. I also don't see how them getting away is really a problem, since that was setup at the start of the fight ("two silvers and one crimson? This'll be over in an instant I gather") and introducing stronger villains makes sense since the only heroes that had any trouble with the previous set were Quna (a Crimson Knight) and Raito, and Raito had gotten the shit kicked out of him before his fight.