The h3h3 case showed that the further you get from just talking over a video in its entirety, to the same run-time, the more likely you are to win a dispute on fair use. Some hypotheticals (ordered from strong to weak):
1. You use clips of Chantal in a much larger video that is discussing her, ideally with some high-effort content included like a skit or animation: this is the h3h3 "quintessential fair use" ruling.
2. You use only sections of her videos and cut away to react to them, but not full videos: fairly safe, although if you're just making a compilation of clips without adding criticism, it may become more shaky.
3. You use an entire video and pause it or cut away at numerous points (the Zachary-style): you're still adding content to the original video, nobody who is only looking for Chantal-produced content would consider going to this over the original because they wouldn't want Zachary's interjections. This is a little harder to defend potentially, but still fair use. Essentially the more you alter the original, the safer it is.
4. You use an entire video and just talk over it, without pausing or adding anything else: this is where it becomes debatable, and would require more court cases to set precedents.
Whether you have Chantal's video permanently full screen or not could affect this as well, as if it's windowed/thumbnailed for a lot of the time, you've further removed reliance on her own production, as most of the screen-space is your own.
