Does quality just make air scrubbers use filters faster, without changing the amount of pollution consumed?

At a minimum, shouldn't the pollution value increase with quality, and be pegged 1:1 with the crafting speed, like we see in most other "fixed ratio" buildings (e.g. turbines, steam engines, thrusters, etc.), or should the "crafting speed" go down with quality instead of up?

Yes.

However, as far as I can tell from combing through the prototype documents, there isn't a way to tell the game that crafting speed should be ignored (or scale down) with quality. It seems the game decides what properties of an entity are affected by quality and how, depending on the type of the entity, while giving no way for mods to interact with that decision.

Under the hood, the air scrubber is a furnace, which is a special type of assembling machine. So the game "improves" its crafting speed (and health) like it does for all furnaces without giving me a way to change how that works.

Which isn't ideal.

If I'm wrong, I'd love to be told how to alter the properties of a machine that quality alters. Or at least some other work around. But until that happens, there's not a lot I can do to fix this issue. I recommend not using quality air scrubbers.

there is a allow_quality = false tag you can place in a recipe.

The allow_quality property is for (dis)allowing the use of quality modules with the recipe. allow_quality = false is already set for filter cleaning recipes and the scrubbing recipes.

It has no effect on the entity properties the game sets on the air scrubber entity.

It looks like there are ways to do it.

This mod seems to show a viable way to change how quality interacts with entities, both in terms of which entity variables are impacted, and to what degree they're impacted: https://mods.factorio.com/mod/more-quality-uses

From there, the question is how you want to implement it, and I can see three ways to do it that seem relatively straightforward:

1. Slow the "craft" time down, while keeping pollution absorption value the same (increasing "productivity")
2. Leave the "craft" time alone, but increase the pollution absorption value (increasing "productivity")
3. Modify both in a way that leaves the relative "value" of unused filters static, but increases the amount pollution each machine can scrub (e.g. craft speed goes up, and pollution absorption goes up with it).

Thank you for the research.

I've added this in version 0.2.0. I opted for option 2, leaving crafting time alone and increasing the pollution reduction.

Due to the limitations in the modding API around quality, it's a bit janky. See the note on the mod information page.