You don't have to prove actual [mask] effectiveness in clinical trials, you can just assert they seem to work and say it's better than not using.
Usefully, there are actual clinical trials that say that masks are good:
https://www.cdc.gov/coronavirus/2019-ncov/science/science-briefs/masking-science-sars-cov2.htmlAnd, surprisingly for the fuzziness you'd expect and the desperate attempts of anti-maxxers to support their theories, very few that show that masks cause any even vaguely significant harm to the general population. Certainly not on a par to their advantages, though certain individuals with specific needs[1] might
[1] Of which there will be some, like those with already fragile breathing capacity[2], but far less thab those who 'need' to be exempt and deliberately muddy the water.
[2] Who should therefore be comforted by other people wearing masks, if not insist upon it!
Regarding Putin, the worrying viewpoint I've heard about the "nudging even more of his troops even tighter up against the borders[3] of Ukraine" is that Putin is smart enough to know that he can't afford to have his bluff called. If he were actually bluffing (to try to force the West to not, certainly from his POV, powergrab territory ever closer to Moscow) and yet still the hint isn't taken then he'd have to follow through on his bluff or lose the face he carefully tries to maintain (both domestically andl internationally). So, even if it's a bluff, it can't turn out to be one. Parallels abound with that time Archie Duke shot an ostrich because he was hungry. Except that in this day and age you're more likely to go straight into a Flash Conflict rather than a fuzzy false-war of mobilisations between various national states. The merest skirmish on the wrong bit of border could quickly ramp things up in a way that, pre-WW1, would have been a 'regretable incident' that was over and done and back to a passive-aggressive stalemate across nominal lines of control, perhaps even before the respective capitals even got word of any exchange of fire, let alone casualties.
Maybe Putin can just get away with it (prior movements into Ukraine, Georgia, etc, suggest that he might), but even the Judo-Chessmaster himself might find that his next "whoops, I did it again" trips just too many alarm signals if he actually goes through with it.
One way to prevent(/defer) it is for the West to fall back and never tempt the bluff into becoming real. But that leaves a bad taste in the mouth (with plenty of recent examples of acquiescing any foot-forward stance, and in several instances seeing Russia stick their own foot into the door in lieu) and at some point even the most mild-mannered and competent Western leadership is going to rail against that. Never mind the less well disciplined types.
[3] Either the real ones, or the ones between the already invaded bits and those not yet done so.