Seems like most of the questions are answered already. I am just going to say if there are specific events/scenes/achievements that are like unique to a character, place, and time.
Kind of like a one time Christmas event for example. And where to get those kind of info. Because, I have looked at the files and there are very specific images that I haven't seen yet while playing the game.
Anyways EraTW is great. If you aren't cheating, I suggest you just go out for some 3 days or so just to find Touhous and get the achievement that gives you the wheareabouts sense perk.
As far as I know there isn't anywhere keeping track of special event stuff. Here's a few things I've found
Suwako will grant you a special perk if you get her to love status.
Momiji will give you something useful if you beat her in danmaku enough times(not sure if you need favor in addition)
If you get Hatate to love she can sometimes help you out if you pass out from lack of energy or end the day with close to 0.
Kosuzu will let you rent a special book if you become friends with her, and will hold on to extra copies of books so you can still choose books that are currently checked out.
Getting to know Mamizou will alter the tanuki drinking party event you can get while traveling.
Getting Seija to love will unlock her hideout as a place you can travel to.
I think if you get friendly with shrine maidens(Like Reimu) you can use special travel spots to go between shrines if they're following you.
Kagerou can visit you at night if she's friendly with you.
Yuuka will give you a plant seed each day if you challenge her to danmaku and choose it as your reward.
Medicine has an event questline involving her getting genitals once you get her to love. Involves convincing other touhous to help her with it. Though you need a certain level of favorability for them to agree(Some are pretty low, some are much higher.) Results of the quest vary on who you picked to help.