This is really how TH2 series ought to be made.
Ikuno-chan's the star in episode 2, quietly making an assessment of all of her sister's potential rivals: Tama-nee the "aneki type", Konomi the "next door imouto type" (kawaii~
), and Ruuko the "eccentric type". Of course, she probably hasn't realized there's seven other rivals with different attributes as well, ranging from tsunderes to time-traveller to twins.
Manaka's story was one of the first of which I played through, just after Konomi and followed by Yuma.
Now if they can, as you guys say, pull off a Comic Party and actually make a series with the same animation production quality, there'll be plenty of happy fans around.
If anyone fancies Ikuno-chan, fret not, for she is one of the new available girls for "girl-magnet" Takaaki to hit on in the TH2 spin-off game Another Days.
About the ED, I didn't even realize that was Rikimaru Noriko singing, her singing voice sounded so much more "mature" than Manaka's "usual" voice.
The timeline of this OVA seems to combine elements from each girl's story and some of the anime. Apart from this explanation, I can't see it as anything other than as a reason to put as many characters together as possible and write an original scenario. Not that I'm complaining though. Maybe it's because of the "normal life" routine of the situation that any scenario is possible, and a potential for milking the franchise is created.