Alright, finally kicking this into gear. General info links to the post of whoever wrote it, rather than me copy-pasting it and claiming it for myself XD
Questions asked in the topic will be re-written up here, as well as the answers (and if you want credit for your question/answer that much, then feel free to say so XD). Also,
please do correct/add to incorrect or incomplete answers! 
The more information, the better.
--
Auditions, Lives, and FestivalsPromotionsPrefecture Map for promotionsCD Sales and Keep/Break/Revival systemsFor quick reference (thank you, Takagi XD):
Do a Fes to get a Break
Do a Live to get a Keep
Do an Audition to get a Revival
Activity SummariesUnit Danketsu/Mood Statuses (and
Correcting Negative Danketsu/Mood Statuses)
Danketsu BreakdownIdol CompatibilityAmulet LocationsJapanese iM@S2 wiki (
W.I.P. English version)
List of
Clothes and
AccessoriesImportant/"Rigged" Festivals:Vs. Ryuuguu Komachi (Week 10) - Forced Loss
Vs. Ryuuguu Komachi (Week 21) - Must Win to Progress
Vs. Touma (Week 32) - Forced Draw
Vs. Jupiter (Week 54) - Must Win to Progress
Story Mode Questions:How can I unlock Ryuuguu Komachi clothes and Ritsuko?Answer the questions asked by Ritsuko in week 8 and week 30 correctly to unlock Ritsuko in S4U.
Week 8: 「アイドル時代?」 & 「すごく魅力的だから」
Week 30: 「そうだよな」 & 「アイドルに戻りたい?」
You can only get Palace of Dragon (the Ryuuguu Komachi outfit) after inviting Ritsuko to a Quintet Live and completing the game.
How many clothes shops are there, and how do you unlock those shops?2 shops (LUCKY RABBIT and BK MANIAC). LUCKY RABBIT is available from the start.
BK MANIAC is unlocked after you spend a certain amount at LUCKY RABBIT.
When does LUCKY RABBIT have sales?Weeks 11~13, 22~24, 33~35, and 42~44.
Is there any way to tell when BK MANIAC will have a sale/when BK will be out of the store?Nope.
Do I have to do Iori's "vending machine" tutorial every time I play the game?No. You can select the right (red) option to refuse Iori-hime's demands, and proceed with the day as usual.
Is there any reason to continue a lesson if it's perfect + bonus? What's the difference from perfect, and what does bonus add?There doesn't seem to be a reason to continue doing the lesson after the bar is maxed out ("bonus" being the confirmation that the bar is as full as it'll get).
What's the difference between a Super Lesson and a Lesson (besides the cost)?A Super Lesson (ちょーレッスン) lasts for a longer time, making it easier to get a perfect lesson.
Also (according to the manual) it gives a better stat-boost than a normal lesson.
Is there a list of all the reporters and what they influence somewhere?安宅 純 (Ataku Jun) increases voltage gauge gain
有明 仁 (Ariake Hitoshi) increases fan numbers by 40%
小柴 徹 (Koshiba Tooru) increases lesson effectiveness
金丸 大蔵 (Kanemaru Taizou) increases money gain by 40%
山原 太郎 (Yamehara Tarou) increases fan numbers and money gain by 20%
Thanks a ton to Cael K. for romanizing the names~~
I'm really confused about what I'm supposed to do to "win" the game. Are there specific events I'm supposed to pass? / How do I get each ending?Bad Ending: No awards won.
Normal Ending: Win other awards, but no Grand Prix award.
Good Ending: Win the Grand Prix award.
True Ending: Win all awards.
How can I gain the Grand Prix award from the Idol Academy?Have at least one song in the top 10 on the charts, and have 300,000 fans.
How can I gain other awards from the Idol Academy?Win 100,000 fans in every area (except Tokyo) and then win the 歌姫・歌王子 Festival that appears.
Why are my idols ditching work / how do I get them to stop?If you use the auto-commands too often, your idols will become fatigued and take a week off on their own accord. Just stop using auto-commands so often (save them for when you actually need them).
Other Questions:Is there a way to edit the idols' singing in S4U mode similar to L4U?No, unfortunately.
How do I unlock Palace of Dragon (Ryuuguu Komachi outfit)?You can only get Palace of Dragon (the Ryuuguu Komachi outfit) after inviting Ritsuko to a Quintet Live and completing the game.
How do I unlock Beyond the Stars (Project Fairy outfit)? (Xbox 360 only)Beat the game with an ex-Project Fairy member (Miki, Hibiki, or Takane) as leader.
PS3-exclusive questions:How do I get the things from the Xbox360's Catalogs 01-03?List here.If it isn't on the list, it's available from the get-go.
How do I unlock Nanairo Botan, Princess Melody, and associated accessories?Nanairo Botan (song): Complete the Extra Episode with any character (also gives you the Ryuuguu no Tsukai amulet)
Princess Melody (Extend): Complete the Extra Episode with all characters (Iori, Ami, Azusa, Ritsuko).
Otohime Extensions: Perfectly complete the Extra Episode with Ritsuko.
Seven-color Bracelet: Perfectly complete the Extra Episode with Iori.
Emblem of Dragon: Perfectly complete the Extra Episode with Azusa.
Ryuuguu Friends: Perfectly complete the Extra Episode with Ami.
How do I unlock Beyond the Stars (Project Fairy outfit)? (PS3 only)Complete the game with all of the ex-Project Fairy members (Miki, Hibiki, and Takane) as leaders.
What does the Ryuuguu no Tsukai amulet do?It gives you a 3-idol burst appeal. It's different than normal bursts, so equip it and go try it out :
D
A BIG thanks to BT2 for pretty much all of the info about the PS3 version!