True. A lot of things could be improved with the design, this is really just to visualize the core concept. I was also thinking a play/pause button on the pinned player would be good.
Getting fancier, maybe double tap on the right of the pinned player to skip forward 15 seconds would be sweet for skipping ad spots, and when you go the original post, the video could be fixed to the top of the page while you scroll into the replies.