[These observations assume you have seen the episode, so there will be spoilers. This is not a summary, just a listing of points of interest.]
I knew it was too good to be true. Finally, a silly episode. Not without its moments, mind, but essentially silly.
Confirmed: Charlotte's combined four girl harem is her Arm. At the moment of Drive, they form a cocoon, which then opens into Charlotte's Butterfly. I note that there was something of a whirlwind effect the first time Mirei Drove Mamori.
Kasumi is indeed blocking some of Charlotte's more petty actions. So does the Governor, whose Arm we get to see, though not very clearly. Who the Arm is, is as yet unknown.
Lady Rein, in Ep. 03, identified the Governor as a "hypocrit" for her speech promising to find a way for the Mermaids to escape their island, but I think this is not entirely true. Both the Governor and Kasumi genuinely care for the girls, and do indeed want to see them all escape--but they are also aware of "those who watch", and know that if they openly support a break, the retribution will be dire.
Kasumi is also interested in, and protective of, the Atelier Torino.
Finally, Kasumi is certain she has seen Meifong before, but cannot quite place her. I suspect Meifong's large-breasted appearance in the Castle is part of a disguise.
Mamori is somewhat simple; I do not say "stupid". Despite being in High School, she acts like she's about ten, at the most. And Mirei is completely undone by her; her affection for the smaller but older girl grows apace.
The girls at the Atelier were also quite taken with Mirei's and Mamori's Drive. But it is not the hard, edgy interest--they were starry eyed romantics, every one. They just loved watching Mirei and Mamori embrace.
Even Kouzuki and Miasato, the slacker C class Knights that tested Mamori and Mirei on the beach in episode 01, seem to be rather fond of each other. Misato applies suntan oil to Kouzuki a little too sensually, and Kouzuki says she's about to Drive--but Misato urges her to hold back, seeming to lead her to a higher ecstasy rather than a transformation.
I like to see the various affections here. The first few eps give the impression of a relentless dictatorship, with the sex being nothing but a way to activate an Arm for a fight, but we now see there is more than that going on with many of the girls.
It was nice to have the change of pace. Nice to be silly for awhile. But I hope we return to the story soon, that this doesn't represent the production team running out of narrative juice.