We're not saying they don't. But let's just say we're still working on it.
There are 7, 8, or maybe even 9 types, depending on how you count dimensions. But let's just say it's complicated.
You can't. That's not a thing. Just store your regular sushi in the fridge, like everyone else.
Only if you're in a pinch, and by "pinch" we mean "a black hole." Otherwise, it's just not worth it.