They say that you can’t get a proper seal with a beard, but I’ve managed to do it before. I wouldn’t be shocked if it did. And if it didn’t a slight leak is may still preferable to complete exposure.

It depends on the type of gas mask, if the presure in the mask is positive, it will work. Air leakage will be high though, because of the beard.

Not sure. One imagines they’re at least somewhat effective to have been in use for so long.