Yes he can breathe underwater and absorb chakra without samehada. Samehada is basically an extension of his power. Basically a way to absorb chakra from a distance without using chakra. Samehada also acts as a container as kisame can use chakra to heal himself, so having stored up chakra in...
Well yes it's a jutsu of the sharingan but it's also an advanced nature transformation. It's literally fire transformed into a stronger form of fire. If it wasn't, it wouldn't be black. Just bcus it is a jutsu of the mamgekyo sharingan doesn't mean it isn't an advanced nature transformation. If...
So when it comes to chakra natures, I try to finish the kkg wheel while keeping it as cannon as possible.
For example Sasuke's novel was considered canon as stated by kishi. So that makes the typhoon kkg canon. The typhoon release was labeled a kkg and not an advanced chakra nature like blaze...