Interesting issue, I haven't seen anyone embed a remote commercial player in Retool before
I assumed that using a custom component would be the best way to do so. What issue did you run into with the custom component? It looks like your comment cut out.
Did you have any luck with the built in Video component? Is that where you hit the issues of passing in {{ get_mp4.data }}&loop=on but the video stops playing when the string changes?