Yep, that's what I thought when I seen that, grenade launcher. My theory on how it works is that it shoots blobs of ink that splatter a wide area and can OSOK enemies, but its fire rate is abysmal. I'm talking like 3-4 seconds to fire another shot. That would balance it out nicely.
Because they're sporty. That's about all the reason needed, really.
The reason I claim braces, though, is that if you look to the edges of her mouth, you can see very very faint protrusions on either side, and the color is ever so slightly off, like a ring of it surrounds her mouth. This makes me think she's sporting retainer braces. (Or at least a stylized version of such.)
Interesting. I think that's referencing squid mouths. They have beaks and that's the best they can do to reference that without it looking bizarre. Pretty cool.. Guess that Inkling anatomy propaganda depicting them to having sharp teeth wasn't lying.
EDIT: Yep, through Google translate, they're definitely referencing squids beak-like mouths. Tweet