First of all, in many cases, writing new code is lot easier than trying to modify/salvage old code from someone else. Unless you can just plug it in for a modular function in that case your code is not useless.
And if they think your code is valuable enough to save that many people after they improve it, they can approach you for dual license or other agreements. They pay people with patent all the time, so they can do the same for people who’s volunteering their time for open source.
That depends on what video player you use. Of we have control of that, then sure it works. I use mpv to play things, so for radio streams or live videos I can go back/forward as long as it’s cached.
But if it’s the web service, even though the browser video player has something cached, the player is still controlled by the website. And considering most of the people use chrome/chromium derivatives or YouTube app, it wouldn’t be hard for them to make it so that the player itself will collaborate with whatever they want to do.
If YouTube was a separate organization it wouldn’t have been the problem it is because of how Google has been taking over all the different parts they need for advertising.