First, there were like 50 shinigami and the menos were coming in separate groups so it was basically about 10 shinigami versus 1 menos, i'd say that still makes them pretty badass

Second, your guess is as good as mine i don't even remember when they started flying i guess it was just something they added for practical reasons

Third, the only reason they attacked the mask was because it was a clean and easy way to finish a hollow, yet going for a cero spitting head when your such a low level shinigami may prove to be a bad idea. However you may notice that the lieutenant does go for the mask since he has enough skill.

I hope this helped.