If...an actual instrumental was released I think it's fine
It's only fine if the official released instrumental was released publicly for free. If it's part of a CD or something, then yeah, some people can cover it and get away with it because the anime company doesn't care, but usually it will result in a copyright strike and audio takedown.
I see...How about the original videos? Does it applies too?
What do you mean original videos? Like Nico Nico Indies? No matter what the genre is, if the instrumental is released publicly and the instrumental maker doesn't prohibit use of the instrumental, then it is probably fine to use.
Depends on how you want to use it. For utattemita, parodies, mashups, choruses,, etc, it's fine. If you just want to reprint it, or use it in a different kind of video (like a homage to len idk) then it depends on the producer, I guess.