In CTB, the "spinners" do not add to your combo. So if a song ends in one of those spinners, when does the song become "FCed". After the last note that gives you combo, or after the spinner?

when the result screen appears

That is for the last map but all the top runs alt+f4 after finishing a map to skip the results screen. I'm referring to the times where we alt+f4 out of the map

How is finishing a map defined? likean00b's current #1 score doesn't actually "finish" the map in that it doesn't submit to the server or generate a score on local ranking, so as far as osu! knows it's actually just a mid-map quit.

I think if the video at least shows the song ending and even though theres a glimpse of the result screen or not, it counts.

Premods aren't really an issue IMO, due to the 1/4 second delay between entering song select and being able to pick a song you can actually select mods without losing any time if you're fast on the keyboard. Whether this should be required as some display of skill is up for debate though.

Seeing results screen is a tricky one, with properly timed escape presses you can skip past it before it becomes visible, while still passing through it so it submits and generates a local score. I prefer this definition myself since otherwise there's no difference between quitting after the last combo-giving note and quitting after the last hitobject ends, but it's certainly not as clear cut to me.

Alt-f4 should definitely not be authorised in my opinion, cause it's not a submitted score, so no results screen to verify. Also it make everything way too tedious, cause you have to control every FC to make sure you got the last hit object, imagine someone doing it for the 50FC run...

Premods in the other end is basically the same thing as having pre-selected you're collection/maps, it don't really see the issue with it.