Well the whole copy thingy has also been explained as not being able to copy Bloodline Limits, such as Haku's, because they use 2 combined elements. If the copy ability could just bypass elemental affinity then Bloodlines such as Ice and Tree/Life would not be a problem to copy.

BTW what elements have we seen Kakashi use? Lightning and Water, but i can't really remember anything else. The hiding in the earth could easily be explained as a regular jutsu not requiring a element. I can't really remember anything else except that. But then again i have the memory of a goldfish so please enlighten me.