It may seem strange that the song title "A-A-Ai" was untranslated until you notice it was written for Ariu Aki. The Japanese title is "AAć„›", where the connection is more obvious.
Most of the couples has some issue that Yuna and Hina have faced or will have to face. Miki has to deal with how to respond to Maki's confession, just like how Yuna responds to Hina's. The broadcasting trio has two of their members falling in love and trying to figure out how not to treat the third as a third wheel, a bit reminiscent of Yuna and Hina falling in love, since both are friends with Ano. Kiri and Tsukuyo's arc has them come to terms with their lesbianism, just like how Yuna comes to terms with hers. Youka and Aki end up drawing some flak after their public display of affection, something Yuna and Hina will have to be careful of in the future. Lastly, Matsuri and Miyu face two important issues- how to keep a relationship going, and how to potentially deal with parental opposition- that Yuna and Hina may face someday. Basically, the entire game can be thought about as Yuna undergoing the necessary development to be ready to go out with Hina, rather than the ghosts learning how to have Their First Time.
While it may seem rather silly for Sachi to be so worried about what Megumi thinks of her real self, consider that she had 50 years without anyone to talk to about her insecurities, and spent the next 30 with Megumi as her only companion, so she'd have ended up in an awkward position if her confession had soured things between them. In fact, the fact that Sachi makes a mountain out of a molehill and continues with her Heroic Self-Deprecation even after Megumi accepts her, can be chalked up to Sachi's vulnerability.
Nena's surname, Miyama, has the "three" of the broadcast trio's Numerical Theme Naming. She's the "third wheel" to Umi and Sasa's relationship. In fact, Ano and Miyama serve the same role to Yuna and Hina, and Umi and Sasa, respectively.
In the flashback to Yuna overhearing her middle school clubmates badmouthing her, it seems a bit strange that all of them refer to the person in question by her first name, rather than her surname, "Sempai," or her title as Club President, even though they seem to dislike her (although the lack of honorifics indicates a lack of respect), but this makes the implication that Yuna had thought of all of those people as her friends. Compare how the people who expect Miki to do their busywork for them call her by her surname.
When Sachi and Megumi are possessing Yuna and Hina to have sex Megumi says that on some level she can't help but wish they could retain the bodies and live together. Thankfully the Kindred Spirits are far too nice to ever actually do this but if they had been less benevolent and more manipulative ghosts Yuna and Hina could have been left trapped in their own minds forever while their lives were stolen by those they'd tried to help. Yuna even acknowledges the possibility in-story before the sex scene.
When you subtract the 30 years since Megumi's death from the 80 years since Sachi's death, that leaves Sachi with about half a century as the only ghost in the school and no other people besides Yuna and to some extent, Ano and Hina who can see and hear ghosts. It's a wonder she didn't Go Mad from the Isolation.
