Bane would win in terms of the sabers, mainly because Kun doesn't show much there but Kun would take him down with the Amulet, probably with ease. If this was RO2 Bane then he would win though.
It doesn't matter. He knows all moves it can perform in all the seven forms, since Kas'Im was a master of them all. Kas'Im himself said that many people think the double lightsaber is really exotic, and they overthink the whole thing, and if you know the location of the other end, you know the location of the other one too.
Except Kun's form died with Kun so no, Kas'im does not know Kun's form, whereas Kun is aware of Bane's form. It's tough to tell which force user is more powerful although I would give it to Kun based on his feats, but Bane is a sucker against forms that are foreign to him.
Because he possessed a double blade with a hilt that was the size of a single blade, allowing him to wield the blade as one would wield a single blade, or a double blade.
Kun's lightsaber had the hilt of a single blade which means Bane would be surprised by the different angles at which he can attack with since the standard dbl's had different angles for the hilt/saber to move. And as Darth Sexy said Kun's form was unique to him and died with him and it has another feature that most other sabers don't have which can also catch Bane off guard as said here:
Millennia ago, the Sith Lord Exar Kun fashioned a modified lightsaber with two blades. During the events of the Great Sith War, Kun modified his existing weapon to increase its lethality. Customized controls allowed him to extend the length of each blade from half a meter to one-and-a-half meters. -Star wars databank
His wouldn't be the only lightsaber that be changed in length besides on and off. If you have more than one crystal in your lightsaber, you can change the length.
No but I'd say that most don't, and even less number of dbl's alter length and his was the only one to be a single bladed hilt so it all adds up to add to a unique form.