You're not going to spend 100% of your day in the coverage of one of these things, so what's the dose response? How many hours per day of filtered air equals how many fewer infections per year? Does air filtration in your workplace matter at all if you have young kids in school? Does air filtration in a school matter at all if the kids are all together on a poorly ventilated bus for two hours a day?

Seems like some evidence would be helpful.

Yes we have studied actual rates of infection in response to interventions like air filtering, so these studies account for all that real-world complexity and messiness you worry about.

The article is complaining that every study doesn't redo the evidence collection, end-to-end, every time. That's not realistic and not necessary.

A lot of your specific questions are leading (with a nothing-we-can-do attitude underneath) or asking the wrong question (eg expecting one universal number for "hours of filtration per infection prevented").

For instance the correct answer may be air filters in classrooms and buses and workplaces, but strangely your line of questioning doesn't even consider that possibility.

This would be like someone in the 1800s questioning how handwashing avoids Cholera if they don't wash their hands at home. I think I see a solution to this one...

Kids don’t transmit tire and brake dust into my lungs