Regarding the lower mesh mask, I bought some single-sided foam (like the anti-draft stuff you get for doors but much thinner) and put it at the top where the bridge is. That helped a lot.
I'm not sure why companies don't just do that and make a rubberised one where it rests on your nose and also further to the sides. I know there are those ones on Firesupport that have fabric on, but they look like they'd get in the way of your nose more than they'd help with comfort.
As for gum shields, I've owned a lot of different ones over time and frankly the fitted ones are far more comfortable (granted you forget you really have on in after 5 minutes) but primarily they're good as they don't like to fall out half as easily as the 'boil and bite' ones. I never wear one during games though because I generally can't stand them, and also it doesn't stop that BB hitting your lips or flying into your mouth. I've seen people using them though so it must be viable.