Update: it works. We're reading the game's framebuffer at native 1080p, 59 fps.
Since the first post, the capture problem is solved. Not through the graphics stack - all three routes there stayed closed - but around it, via the kernel debug...
Repo: https://github.com/TomasUribe/switch-frame-tap (GPL-2.0)
What I'm trying to build: a homebrew sysmodule that streams the Switch's screen to any PC over a USB cable at the game's native resolution — 1080p, 60 fps — with no capture card and...