@nekohayo After installing "gstreamer1.0-fdkaac" I can now play the first 20 seconds of some YouTube videos before epiphany stalls out... so it looks like I am now hitting one of the bugs you linked:
https://bugs.webkit.org/show_bug.cgi?id=289194
What's interesting is that I can play ads completely fine (and they are sadly not skippable).
Some progress, I guess.