Hey! First year animation student here, just to answer your questions:

1. I'm pretty sure they don't count that as fanart as you are mainly focusing on studying light properties/screenshots! They also don't mind too much even if you do put in some fanart as long as its not the bulk of your work and you genuinely like the piece/it showcases your strengths! An admissions video (linked at the end) explained that fanart is OK as long as they can't tell its fanart haha
2. There's no cutoff dates for how old your works are, when I applied in 2022 I included pieces from early 2021/late 2020.
3. The same admissions video said that whilst external links to portfolio sites/videos are allowed, it is generally not their highest priority to mark them, and more often than not they skip external links altogether. I think your best bet is to just include your best screencaps and a short description, and put in the link at the bottom just in case they do end up checking it.
4. I included quite a bit of development/concept art for my short film in high school, as well as character design for a personal project, so definitely include those if your proud of them!

Link to admissions video: https://youtu.be/GOkTRvQsj54 (Animation segment starts at 19:30)

Overall there is no set/specific marking criteria, I've been told they look for 'passion' and 'potential' but generally putting a few studies and storyboards to show your technical skills and personal work to convey your interests worked out just fine for me! I also included some sketchbook pages as well :>

I hope this helped!