I only know about masks in what I read...not trained in N95's. But the concept of not having a leaking mask is familiar to me as part of our dive team and scuba.
In my early research on N95masks I found this video...it addresses many aspects of masks...filtration levels/designations, mask types, etc. At 7:15 it talks about the "fit" of the mask. You are right. Many folks do not fit the mask properly to their face. Even when I first used my 3M half face mask to remove fiberglass months ago, on the first day it didn't seem "right". I realized I had not fastened the lower neck strap and there was some open space. The mask has a soft seal that is excellent...when worn properly. I figured out with my N95 mask that I can cover the filters with the palm of my hand and inhale. The mask should suck down onto the face and not allow any air to enter around the seal. We do similar with scuba masks when fitting them. (see 9:40 mark on video)
This also has some good info.
https://www.cnn.com/2020/04/01/asia/coronavirus-mask-messaging-intl-hnk/index.html
There is another article I cannot find right now about virus size and droplet size. It said that basically some non-N95 masks (dust only) might stop large sneeze droplets..the article discussed how small the actual virus is (you are right) and how small the simply exhaled droplets and virus are. It takes an N95 (or better) to filter out those small droplets.
If the mask does not fit properly the droplets go around the edge.
The mask can contain sneezed and viral laced exhalations from YOU so they can protect others. When I picked up my mother's tax papers at the accountant office yesterday, they had a person bringing the packets to cars, you sign for receipt and they carry on. It's a good attempt to minimize exposure but the girl had no masks or gloves. Is she contagious? No one knows WHO is infected. The latest I have read says lots of folks are asymptomatic (NO symptoms, or maybe they think they are having pollen allergies) and they are shedding the virus. I masked up before she got to my truck...figure it will protect her and me since we could either (or both) be infected. And I did glove up too....she was running a credit card thingy...has she been taking cards from someone that's infected...and she has it on her hands, etc. It's challenging...and a real mess. Hard to fight unseen enemies.
My martial arts teacher taught many "systems"....Animal, Edged Weapons, Stick, Grappling, etc. One was the Wizard System...he taught this system starting in May 2011. I studied it with him each week for 8 years until he moved away to Baltimore last October. This System is built upon the premise of fighting the demons we all experience...both outside and inside ourselves. This is how he described the Premise of the Wizard System....it resonates almost too well with what we all face right now:
Premises of the Wizard System
1. One of the ancient Weiza [wizard] beliefs was that man’s eternal enemy is the demon [evil spirit, the devil, Satan, etc.] that continuously attempts to destroy peace and harmony of humans, animals and plants.
2. The concept of "demon" symbolizes poisonous, venomous, toxic, negative and destructive forces that cause psychic and physical harm.
3. The goal of the demon is to capture human souls* and enslave them in the underworld for eternity. *[Difficult to translate the concept of soul.]
4. The demon is sly, crafty, skillful and cunning. It will change its shape and form** to persuade, seduce, entice, deceive and manipulate the minds and emotions, and finally to capture the human soul. It will attack with speed and stealth from the front or the back, from side to side, or from above or below. **[Demons can change their shape into human, animal or nature's forms.]
5. The demon can become invisible. It can attack with speed and surprise from the front, behind, sides, above or below. It only becomes visible when it attacks. Otherwise, it remains invisible to the human eye.
6. The demon can be defeated, but never killed or destroyed. It will put on different disguises and return again and again at unexpected times and places to disrupt the harmony of mankind.
I bolded #5...that is the insidious nature of the coronavirus that has so many concerned.