because kakashi just coppies them.he was able to use water dragon because he was standing in the water.he just coppied the hand seales and everythink happened automatically.he uses 2 elements for real:lightning and fire.he wont be able to perform a water teqnique away from water cause he is not able to turn his chakra into water.its simple
hell no!!!!!!! no katon(fire).
YET!!!!!! show me any refernce in manga, only then i will acknowledge it. i wont count anime, those are made up by animators, not kishi. did u forgot kakashi uses doton(earth) all the time?(latest was against pain) so far he uses : lightning, earth, water.
ppl have affinity for
only one element(remember the chakra paper test, it tests ppl's chakra affinity and it gives only one result out of 5 for every person), then they pick up more elements as their skill grows. but other elemental jutsu wont have the same level as the one with affinity. that is kakashi's doton or suiton jutsu wont have the same level what his lightning jutsu has.
abt water, kakashi spit water during kakuzu fight when their was no water around. bcos it was not a higher level jutsu i suppose. but true, any other higher level jutsu like water dragon, he wont be able to do without water around.