How exactly did Chihiro know Himura before the accident?
Yuu rented living space from Chihiro and Kei's grandfather when he was in high school, same as Hiro. Yuu's studio is, I believe, the one right next to the one Hiro gets in Miyako's chapter.
Call me bitter about the crushing sadness and unfairness of it all but seriously: Yuko got hit by a car (one of perhaps a dozen we've seen in the entire town) which didn't stop (ok, hit and runs do happen), and you're telling me that there was NO-ONE remotely close who could have helped her (this was, remember, next to a CHURCH on CHRISTMAS EVE), instead leaving her to drag her broken body off the road and die slumped against a tree. Damn you, Diabolus! Damn you!
It was the middle of the day when she got hit by that car, at least in-game, and Yuuko died from blood loss very shortly after being hit; since she's already dead when Yuu shows up, even if an ambulance had been called it probably would have been too late.
So... did Chihiro regain her memories (or at least the ability to form new ones)? The symbolism of the scene suggested to me that she did, but it was pretty vague... Does the original game clarify this any better?
She does not. The ending of her story in the game is completely different. In-game, Chihiro talks about how she can remember things for an extra week after she should have forgotten them as long as she thinks about them frequently.
Don't know about the anime, but in the game, basically it was implied that even if she is unable to form new memories, she is capable of character development, because she doesn't "reboot" at any point. For example, she falls in love with Renji, for whatever reason, 5 hours later she will still feel that she is in love, and another 10 hours later, she might forget what was the reason, or even who is Renji but she will continuously remember the feeling of love as she felt 10 hours ago, and so on for an indefinite time. She can do the same thing with memories, like facts, or pictures, but unlike emotions, they are not naturally continuous, so will become copies of the copies, and still disappear after a few days.
To this troper's understanding, she did not. The shattering chain in the anime was symbolic of her overcoming her condition, rather than the condition disappearing. I also seem to recall a scene in Melodies where she says she can never change but she loves Renji so she will keep on trying.
She hasn't rid herself of her condition, and never will. It's just that now she has someone she can rely on who actually wants to be with her, as opposed to Himura who was taking care of her out of courtesy. Yes, she will never be fully independent, but for her it's still a happy ending as it is possible for her to live a happy life.
You'd think someone as pretty and obsessed with being loved (and seemingly willing to settle for anyone who will love her) as Miyako would have been able to get a boyfriend by the time she was in her second year of highschool.
That's the thing. She was never at school. She always ditched because she thought it was boring. The reason she picked Hiro is because he had the time to be with her.
The plot takes place in Japan, and some things there are pretty different from what we know. What she needed to get close to someone was a trigger, since most of her classmates just saw her as "that girl who has good grades but is almost never at school", and thus were reluctant to talk to her, and Hiro is the first person who actually had a reason to associate with her (even if that was getting back the money she owed him for the bike); being one who didn't care to stop her from associating with him because he was too busy with his life, he also quickly came to accept her presence as something quasi-natural, which allowed her to find her place at his side. Even in real life people can be weird like that, though situations like that one are kind of uncommon.
So wait... If one town is in Japan and the other one is in Australia, how is it that all the characters are able to talk to each other so well? I mean as far as I can tell, Renji and his mom grew up in Australia, but he is easily able to speak to Kuze, Mizuki, and Chihiro. I mean it is possible that they are all speaking Japanese to each other (like in the sub), or it is possible everyone in the main cast learned a second language. It is just weird that this is never really addressed.
We all know that the Otowa in Japan was hit by a massive earthquake; and in the aftermath of such a huge disaster, other countries lent their help to rebuild the town, Australia included. As a gesture of gratitude for that aid, the local government of Otowa decided to establish a massive "sister city"-like cultural exchange program that includes the following: establishing the Australian Otowa from a piece of land donated by the Australian government, mandatory English and Japanese language classes for the schools in both towns (this will answer the "second language" question), and worry-free immigration from either Otowa.