And now it's broken again, and I'm softlocked at 60fps, and I don't why. I've been controlling it for the last 3 days, but today it doesn't work anymore. I'm stunned, but this is just Elite: Dangerous in a nutshell.
I'm not sure you can use any of my advice. Sorry.
I have no sensible advice I can offer, but there must be something fundamentally wrong with some component in your VR render pipeline. The only thing that comes to mind is the frame rate limit in ED's settings, but that should only affect the mirror window, not VR. Did you accidently set your G2's refresh rate to 60 Hz? Although, you would notice. It flickers horribly.
And for the record, I have never had the problem of foreground windows or ED's window focus eating up performance. It makes zero difference on my system. If I were in your shoes, I would probably try to purge WMR, SteamVR and Mixed Reality for SteamVR from my system and start over.
As said above, the one thing you could check is the refresh rate in the WMR settings. You can set it to 90 Hz or 60 Hz, but that flickers horrible - you would know. Also make sure you have set everything in WMR to best visual quality and don't let it adjust anything for performance (sorry, I don't remember how it is called in the WMR settings, my PC is off).
Other than that: Sorry. This really sucks.