Really the only safe way is to have their ark open right in front of the main gate to HQ. Though its been a while do remember that thing scanning Allen and picking up his curse. So assuming that wasn’t just a joke should be able to detect a Noah being different from a normal human. So that way Lulubell couldn’t pull the same trick twice. The canal route doesn’t have any such defence and with her ability to transform it wasn’t tough for Lulubell to infiltrate. Guess it took time to open the gate and leave for the same reason that she didn’t just open the gate immediately. Probably took some time to make preperations.

Episode pretty much just the crazed response that HQ is going through after being completely caught off guard. Getting exorcists who are injured or have weapons in need of repair into secure locations and getting the Generals out on the move. Allen is just annoying them right now since it isn’t really slowing down the Skull transformations or stopping Lulubell from taking the Akuma Plant. Which they can’t afford to lose after all the time and effort put into getting the thing.