What I found noteworthy in the video you linked to of the scene where Deadpool's costume is slowly taking shape, is the fact that right after Wade Wilson asks the audience
Is the mask muffling my voice?
he changes the material, from something nylon-like to something sturdier, which looks like carbon fiber. Since carbon fiber has high stiffness, and can be woven or perforated in such a way that it allows for ease of breathing and talking, that would be a realistic explanation of that part of the question.
Now, to attempt to understand the mechanics of the eye-part of the mask:
In this video we are offered some insight into the making of the mask, and we are presented the following close-up of one of the iterations:
We can see that the eyes are covered with a kind of gauze, which practically act almost like a one-way mirror: it's easy to see through when it's draped across your eyes, but from afar it just looks like a white haze.
As for the expressions, I refer to the answer you mentioned in your post: it's a way of making the character more appealing and believable (paradoxically) to the audience, despite the improbability of a mask actually being that expressive. I don't think Wade would put the time, effort, and money into making his facial expressions more obvious for the villains he crosses paths with, even though he characteristically likes to fool around physically and verbally. And it's also good to remember the mask is based on that of a character from a comic that was ultimately an action hero parody.