If it helps, you can take this as my permission in writing as of date/time stamp of this post, any channel that has taken part in this challenge so far has my permission to use my Sunny Bear 2018 content to make a "mash up" video for the purposes of their "R - Reflect: make a montage of your favorite bits from some of the other people's entries in the challenge and give a shoutout to each person that entered (even if they only made one video)" entry. I imagine any clips used by anyone would fall under fair use and will only be short clips rather than complete videos it'll not be an issue.