Hey - Just reading all this, I'm a bit confused why changing from /videos/ to /video/
If you go to some of the biggest sites [ examples - xhamster & faphouse and even Reddit all use /videos/ ] - And have actually gained traffic per SemRush.
So I'm not too sure that was the issue - I have also not experienced any deindexed and I use /videos/
I did experience some issues with 1 video site a few months back - after doing some digging around, it was advised I remove any ads at the top of my page and allow the video player to fully load before the fold of the site. So I did - I had 1 ad at the top that would appear before the video player and removed it - I also moved the title, date, user to below the video as well - I saw improvements within a few weeks.
If you go over to Twitter and search "google deindexed videos" You will see 100s of people who have experienced their videos getting deindex from all sorts of niches, sports, entertainment, podcasts - starting in October 2023 - each person has a "different" reason as to why and how they tried to fix it.
I just wanted to say this, as I don't believe the deindex issue is specific to KVS.