I like this idea! clever and useful!
2. The very same "button" should be optional available in full-screen mode.
It is. The VLC interface is customizable (except for Mac). You can add this button to the fullscreen mode UI with this function.
I understand that there is no universal or reasonable method to write cross-platorm code that consistently creates consistent UIs. And in order to result this, Mac was given a static UI that hasn't really changed since first implemented. Perhaps it might be a good idea to check if the technology and/or available APIs/frameworks has progressed to the point that perhaps unifying the UIs would be more feasible? I know of more and more cross-platform apps which have complicated and sophisticated UIs which seems identical across operating systems.