Never thought about using an actual WebExtension api (I am in an addon now after all, not simply in userscript) to remove the excess history items, it was very simple to do and worked flawlessly, thanks!

Next update today or tomorrow with Twitch VOD's and this great workaround (along with other very minor changes).

Edit:
update released, now it's perfect

> restart the browser and have 20 half watched videos
YES, exact reason I made this in the first place, as userscript at first and then an addon so I have it anywhere