Initially I can say this: I think your definition of plagiarism is inaccurate, impractical and even to a certain degree intellectually dishonest, for you know most of the "taking the work of others and passing it as your own"(plagiarism) in art is done by reproduction/rephrasing and not by outright art theft.
Saying only art theft is plagiarism is as obtuse as saying its only murder if done against a wall and with a 7.62 nato round.

It is a sort of argumentative Synecdoche fallacy (https://en.wikipedia.org/wiki/Synecdoche) where the part is taken by the whole or the whole for the part, in this case you take plagiarism to mean only a very reduced part of plagiarism, namely, straight out art theft is plagiarism, but there's obviously much more to it....

I will reply further later.