Thank you for this topic, jlgrall.
I was going to suggest you add a comment to Support slow motion for transcription #1820 on GitHub, but you have already done so. This possibility to change the video speed would indeed be most useful: already via accessing the commands of the YouTube video, but it would be even better if it could be done via a native Amara option, so that it also works with non-YouTube videos.
Sure, thanks for the answer !
I found the following objects with properties "play" and "pause":
Popcorn.p // That must be the Popcorn prototype Popcorn.instances Popcorn.instances.media Popcorn.instances.video Popcorn.instances.data.trackEvents.parent
But they seem to be only proxies to the real objects. Thus, I had no luck trying to change the playbackRate, nor trying to show the youtube controls...