Charles de Lorme, who popularized the full costume, lived to be 94. This was not however, actually indicative of the mask actually working. Most people at the time even believed that the plague doctors were useless.

At the time, germ theory wasn't completely understood and the Miasma theory was popular. The general belief was that using herbs could counter the "miasma" that one would breathe in. We now know that transmission doesn't actually work as such. It is also believed that most plague doctors did more harm than good. Travelling from town to town and having contact with mostly plague victim, they played some part in help to transport and spreading the plague with them. There's no indication of plague doctors having a lower mortality rate

It would depend on the germ, but anything that might interfere with the transmission of an airborn pathogen certainly wouldn't hurt. For the Black Death, the pneumonic version was actually relatively hard to acquire and rare, though it was by far the most lethal. Insects---fleas on rodents---were the largest vector for disease. As far as I'm aware, for the medieval period, the data that we would need to talk about death rates in such a specific section of the population simply doesn't exist, and therefore it's hard to draw any other conclusions.

For more on the pathogen behind the Black Death, see Haensch et al., "Distinct Clones of Y. Pestis caused the Black Death," in PLoS Pathogens 6:10 (2010), and for general reading, see (now a bit out of date) Ole Benedictow's The Black Death.