when not distancing, like on a cramped airplane, a mask is better than nothing and is doing the work. in that scenario, any mask is better than no mask.
there is no multiplying going on.
transmission is primarily a function of time and distance, which is why imperfect is probabalistically ok. we've already covered distance. passing someone around a corner is a miniscule additional risk because of the time factor. just don't stop to shoot the shit and you'll be fine.
It turns out that wearing a simple surgical face mask dramatically reduces the detection of coronavirus in a closed laboratory setting over a period of 30 minutes (1). It also turns out that social distancing alone works too (2). Both reduce the R0 (reproduction number) of a disease. Masks reduce the "contagiousness" because fewer infectious droplets make it out into the open. Social distancing and quarantine measures reduce the likelihood of infectious transmission between infected and healthy individuals.
This is multiplicative.
For the sake of easy numbers, let's assume that wearing a mask captures 90% of infectious droplets and that every meter of distance results in 60% of the droplets in the air to fall to the ground. If a sick individual wearing a mask coughs out 100 starting droplets and is 2 meters away from a healthy individual, the healthy individual will be breathing in 2.5 infectious droplets (100 * 0.10 * 0.50^2).
This is multiplicative.
For what it's worth, in real-world studies, when the use of face masks and/or social distancing became enforced, the rate at which infectious transmission occurred saw a measurable drop. The introduction of mandates requiring face masks (with social distancing already in effect) saw a 2% decline in the daily growth rate of COVID (3). If you'd like to read more, there is a recent meta-paper that surveys 172 different studies and concludes that:
"The findings of this systematic review of 172 studies (44 comparative studies; n=25 697 patients) on COVID-19, SARS, and MERS provide the best available evidence that current policies of at least 1 m physical distancing are associated with a large reduction in infection, and distances of 2 m might be more effective. These data also suggest that wearing face masks protects people (both health-care workers and the general public) against infection by these coronaviruses, and that eye protection could confer additional benefit. However, none of these interventions afforded complete protection from infection, and their optimum role might need risk assessment and several contextual considerations. No randomised trials were identified for these interventions in COVID-19, SARS, or MERS." (4)
1: https://www.nature.com/articles/s41591-020-0843-2
2: https://www.thelancet.com/journals/laninf/article/PIIS1473-3...
3: https://www.healthaffairs.org/doi/10.1377/hlthaff.2020.00818
4: https://www.thelancet.com/journals/lancet/article/PIIS0140-6...
[1] limited, controlled experiment =/= real-world effect. every researcher knows this (even if they sometimes don't want to admit it). at best, you have a line of inquiry to follow, not a conclusive policy measure to enact and enforce.
> "For the sake of easy numbers..."
frictionless point-masses are pretty amazing too. an idealized 2d planar diffusion model ignores the very important 3rd dimension, not to mention a large part of the time dimension too (as well as many other significant variances in the real world).
> "...will be breathing in..."
potential exposure =/= ingestion =/= infection
> "...in real-world studies, when the use of face masks and/or social distancing... 2% decline..."
confounding explanatory variables. it's the distancing, yo.
that study summary [3] even says "estimates suggest..." up to 2% (at a population level), not we found undeniable and replicable effects across a variety of conditions that control for confounding measures.
[4] "...could confer..." == we don't know yet. it's the distancing, money!
even your selectively chosen citations show that research has yet to reach conclusive positions on the role of masks on real-world effects (and they don't contradict the idea that masks may be additive but marginal), and certainly not lockdowns. they do show decent evidence that distancing works as the primary mitigation (at the very least, not contradicting that position).
FWIW...
'Yuen Kwok-yung, a professor of infectious disease at the University of Hong Kong, has advised his government that all social distancing can be relaxed – but only if people wear masks in enclosed spaces such as on trains and at work, and that no food or drink are consumed at concerts and cinemas.'
https://www.theguardian.com/world/2020/may/22/why-we-might-n...
(Yuen Kwok-yung is one of, if not the leading infectious disease specialist in Asia among other things, and has his name on many peer reviewed scientific papers.)