The app is working if directly connected to the server (no ssh tunnel, same network); is the playback function on the app using UDP by any case? This would be enough to justify why it doesn't work through the ssh tunnel (which only passess TCP through). The apache proxy seems to have no noticeable effect; browser works with or without it and app just doesn't, regardless.
I will send you a debug log about the playback on the browser; would it be possible for the app to fall back to the same method the browser uses if the first try goes wrong? I guess mine is not a common setup, but it is definitely a secure option for remote viewing (given you have a fast enough internet connection of course); will it evere be supported?