Yes, there is a minimum difference for critical hits, not sure how much it is but its definitely there.
Blocks reduce the hit to the lowest possible damage recievable. This includes criticals.
Elementing the suit doesn't lower the ciritals and more seeing as your already recieving the lowest possible damage. 80% sure thats correct.
Maximum block rate is 96%, not sure about evade but you can get over 50%. Pretty sure block rate doesn't change for different level gap mobs but evasion does.
Rate of recieveing a 0 from same elemented suit, no idea xD. But i'd assume that it gets higher the higher the element is upgraded.
I don't know if the game has an 'Improbability Check'. Probably not but meh. The chance of recieving over 3 criticals in a row is rather small.
For finding out your damage recieved etc use Jon's Charachter Simulator. Its pretty amazing

You can probably find it if you do a search on the forums or google. It pays to download it, rather small, under a meg as its a whole lot faster and more detailed than the online one.