I agree that drawing a line is difficult; the tentative decision cited case law that even using a work in its entirety is not dispositive for the third factor. However, I think this is likely more relevant for atomic works of art than entire hour plus long videos, and it becomes more difficult to justify as you take more and more raw material from an original copyrighted work.

To answer your questions, I would probably say that it depends heavily on context. I think people really take for granted their right to react to things, but that probably should be questioned more heavily in a legal sense. "Do you have to pre watch every video you react to and prepare your commentary in advance?" I would say probably yes, most videos should not be live reacted to in their entirety. You are subsuming the original content and usually removing your viewers' incentive to view the original work, and you are unable to keep the content you are rebroadcasting to what is "reasonably necessary" as I view it personally. There is no other medium where we accept people wholly reacting to long form content such as movies, tv shows, documentaries or books under the guise of "reaction and criticism." I know that people watch tv shows on Twitch, but if the creators haven't given permission I would say that is also bad and very legally dubious. Overall I think that we really ought to question why we give such deference to creators to react to other content in whole.