So...That's a LOT of questions. I'm going to focus on the easy ones for now. One thing to be aware of is that the game as a whole is more of a sandbox, than a definitive storyline. You likely have noticed that it doesn't particularly advance beyond Freshman year at the moment.
1) Yeah, once you complete the strike & get everyone's pay up to $15, the storyline is concluded. There are likely individual scenes you have not experienced, but the storyline is done, at least until more is thought up & added.
2) I actually have not particularly advanced the River Rat past Senior Server, but, arguably even more so than Quickieburger, there isn't particularly much 'storyline' here. It's a place to earn money & potentially have wild times occur.
3) More opportunities for Debauchery mostly. The Party Popularity stat you might have seen mention of increases WILL be relevant to the next big patch currently being worked on by Anthaum, but that will go live when it goes live (i.e. when it's ready). The Quad party exists so that the MC can blow off steam & recuperate for the next week of school.
4) So, 'Favorite Date' is something that applies to any NPC the MC is getting serious with. Basically, it just means the there is one specific date option they like best. For NPC's with a particular personality, that 'favorite' date, might not even technically be a date. Sometimes it's a booty call. Note however, unless it's a booty call, it WILL be a DATE, not a hangout. I don't know if the 'coffee date' option has become a favorite date for any NPC's, however.
5 & 6) This question is confusing. I'm going to add 20 since they are all somewhat interrelated. Each sport has varying degrees of storyline associated. Football & Cheerleading are interrelated & basically just free-use cheerleaders after winning games, with a sub-'plot' of intra scholastic rivalry with another school's football team. The 'being filmed having sex in a hotel room' scene is a plot by the rival team to demoralize your team mid-season. It happens after winter break generally & only happens if you are on either Football or Cheerleading. The swim team storyline is basically turning the swim team into a sex cult (I said what I said). It continues well past naked swimming, keep going, sex will happen.
7) There is no 'storyline' per se for the Arcade. It's an opportunity to make money & have uncomplicated sex once your Disinhibition & Exhibitionism are high enough. (they don't have to be that high, either).
8 & 9) Again, there is no plotline associated with Streaming. There are a handful of scenes that happen if you stream, but no plotline. If you haven't already started naughty streaming, once you hit about 50% streaming popularity, you get an e-mail inviting you to start doing so using the same handle you use for regular streaming. If you take it, your first Naughty stream will get a lot more traffic. Streaming is one of the most reliable means of improving your Dancing skill (don't get me started on that rant, we don't have that much time...)
10) I don't have answers to this one, I avoid that NPC like the plague. (being their sub is not on my to-do list)
11 & 12) The Admirer eventually invites you to their RPG sessions in the Quad mornings or afternoons (if you go in the morning before classes, there won't be one that afternoon), this eventually culminates in a scene in the game where you can push the relationship with them into D/s, & you can choose to be either D or s. The Harasser relationship is complicated & multi-layered, because it will affect scenes outside of the classroom & in unexpected times & places. If you go along with them, you will find yourself more & more submitting to them, with the possibility of eventually becoming their submissive. If you don't, and are so inclined, you can flip the script on them & make them your submissive, but they are still pretty dominant-leaning, so they will be less 'submissive' & more... 'bratty', I guess.
13) At present, there are no events specific to any particular classroom buildings, no. Any events that can take place on-campus, can still occur there however.
14) Teachers/Faculty are ALWAYS going to say sexual/romantic behavior is inappropriate, at least where they might be observed. Interact with them in their office. You will get a message at the end of the interaction that will let you know if there is a point to trying to get sexual with them. IF the message says something along the lines of 'you don't think you are even their type' That means they aren't interested, yes.
15) Extra Credit scenes only happen from Extra Credit. You can visit the Clinic now for condoms/other forms of birth control, but that's it.
16) That may be it for the moment. The Library scenes are some of the newer ones, so there isn't as much variety as some others.
17, 18, 19) Those I don't have answers for, sorry.