Agnosticism is untenable in some scenarios, sure.
E.g., when you have an oppressive status quo, and the options are to resist it and argue for change, or accept the ways things are, and thereby support them, then there is no agnostic position. This isn't a game: sometimes you simply don't get to sit it out.
> and inherently precludes the possibility of nihilism
I am quite happy to preclude nihilism entirely.
> It would be extraordinary - and flatly in tension with common sense - to believe/claim that agnosticism and/or nihilism were not live options for some domain.
I could give a lot of answers to this, but I think the simplest one is that what you regard as "common sense" is not necessarily the same as what I do, so I don't find that a very convincing counterargument.
We aren't talking about whether agnosticism and/or nihilism are unreasonable. We're talking about whether they are impossible tout court.
It's an old tactic the one you're taking, which is to carry off your arguing partner into the weeds of oratorical flourishes and pseudorationalist blow-by-blow point-making. One can make any point they like "tout court", that doesn't mean I am required (or indeed that any useful purpose is served) to spend any mental energy formulating a substantive reply.
Your argument seems to be that there are only ever two possible political positions: pro-status-quo or anti-status-quo. Is it really that simple/non-complex?
This has always been a simplistic take on disagreement, action, and belief. It deems agnosticism untenable and inherently precludes the possibility of nihilism.
It would be extraordinary - and flatly in tension with common sense - to believe/claim that agnosticism and/or nihilism were not live options for some domain.