So a bit of an update on part of my questions: A work colleague of mine with a 2chip PAL SNES got an OSSC, and tried the reverse LPF (in Line4X mode), and the result wasn’t perfect but looks kind of ok: https://photos.app.goo.gl/NHTarHUVkSCV0ibf1
I then borrowed his Packapunch sync-on-luma RGB cable to test the theory of whether my cheap cable could be making the reverse LPF perform worse than expected. And while there was a noticeable improvement in video quality (solid colours instead of checkerboard patterns) the reverse LPF seems to behave the same as before, with no settings being able to make it look much better. Here’s some close-ups in Line5X:
90 degrees sample phase: https://photos.app.goo.gl/ZEA1r5BhIwlzm5e73
180 degrees sample phase: https://photos.app.goo.gl/s0NVeR9mnZcVKOfJ3
So I am now fairly convinced that there must be quite a variation of the amount of blurriness applied for different 2chip consoles.