First, thanks for the info on HEVC ! I really have no idea how it behaves. So now I have a new hypothesis:
Maybe we should never use the $1 HEVC, because it contains the "soft decoding".
Right now, when you use "pure hardware" HEVC for 7k videos, the playback will first fail and stop, but then once you click on "play" again, the hardware decoding will work. You will get smooth playback from there. But for the $1 HEVC, it will always resort to "software decoding", so you will never get it right.
Further more, once you use the $1 HEVC, it will not downgrade to "pure hardware", unless you did a completely uninstall and reg cleaning. This is why HEVC problem is so hard to fix.
I think the playback problem in Oculus native app is due to its built-in codec. Maybe you should try encode the 5k video clip in H264/8bit, HEVC/8bit, then HEVC/10bit formats, then compares their performance.
Also, for 4k HEVC videos, your system shouldn't have such high numbers. If that's GPU decoding, then CPU should use only 5% or less, if that's CPU decoding, then GPU should be at %5 or less. Now are you saying when decoding a mere 4k video, your CPU is running at 80% while GPU at 100%? That's very strange. Even purely CPU decoding, it shouldn't go above 50% for 4k videos. And for GPU, it should be impossible to go 100% for a 4k video.