Demon,
As I read throught the thread I did not see where you had bypassed the filter using the gravity feed option. So that certainly rules out the filter.
Way past my paygrade, but it sure seems like something is causing the idle circuit to go nuts. But what? That is the big question.
You did say plugs indicate rich. So what can you do to lean it out? Rich charge burns faster, so at idle that can not be good. Once off idle all is OK. So idle circuit is the focus. I am no carb expert, but what does one do to lean out an idle circuit? When I watched the video it does not appear that changing the idle screws had much impact. Just like many throwing stuff out there.