Colloquial words come and go - it's a fashion! I remember the word "filth" (=''great', 'super cool') going around at school, then ten years later it came back again. "Sick" was replaced with "ill" as a superlative recently, but thankfully that's passed. Tired of hearing people say things like "this band is 'iller' than that band". If I wanted to say something is "cooler than cool", then why not say it's "Antarctic"? ;)

I hear a lot of young, cool, hip people say something is "off the hook." I'm not young, cool or hip, so I don't use that expression, but it is similar to cool. "That movie was off the hook." (informal /slang)

yes. "cooler" is cooler than "cool". But there is only one that is the "coolest".

Well cold is cooler than cool but I assume you are referring to the colloquial usage. Depends on your age and location as well as the context but here are some I know: wicked, ace, uber, sick, awesome, brilliant.