Very strange, I used OBS myself to record footage for the trailer. Are you sure you clicked "Resize output (source size)" by right clicking on the video source in OBS? If not that, I'm afraid I have no idea what might be causing the problem. Apologies about the mouse thing as well, I'll change that in an update. For now you can press ESC to bring up the pause menu and that will untether the mouse from the screen.
I'm glad you had fun, despite the mishap.