It's very dangerous. There's a good chance your face may explode from the pressure that's unable to escape if you choose to do this. If you decide to do this, I'm coming up with a 32.33 repeating of course, chance of survival.
Only if you're capable of breathing through your nose.
Personally, I can sometimes nose breathe and sometimes can't since I have random allergy flair-ups throughout the day. I make sure to mew whenever I get the chance, but a lot of the time I have to mouth breathe. If you can never breathe through your nose, then you will need to see a doctor and get it sorted out. There are a lot of things that could cause problems with nose breathing: allergies, nasal deformity, nasal polyps, or just extreme maxillary retrusion.
Another thing is, it's possible to breathe through your nose without Mewing, but this won't give you the full benefit. You'll have to do this if your palate is too narrow to fit your tongue on it. In that case, you're pretty much screwed unless you get palate expansion.