The Problem still exists, but I might have find somehow a workaround for the moment. I wanted to solve my second problem, stopping the video when I hit the close button. So I found this post here:
It works and not only that, the video thumbnail shows up correctly, not blurry and with the play button.
But again, it is only a workaround for my case, the bug I explained in the first post still exists. It seems to be fine on android phones, but not in iOS Safari. I hope someone of the Webflow team reads this and finds a solution. Thanks!
Cheers
Florian