kakashi the copy ninja.....in possession of thousands of jutsus.....and some people think he needed naruto to learn kage bunshin? even the closet pervert knew kage bunshin! well, according to the anime anyway
also i dont think the gates thing is difficult for jounins to learn....if anything its prolly just difficult to train your body for. When kakashi was impressed about lee doing it, i think it was because he could reach 5 gates and he's only a genin, but i'd be surprised if kakashi didnt already know how to open at least a few gates.

kakashi learning the mange sharingan is like copying advanced bloodlines: he cant do it. One possibility is that he's not uchiha so sharingan technically isnt his bloodline, even though he can use it, another is that the mange is just simply a higher bloodline limit than what he's capable of reaching.....but either way im convinced that its not something he'd be capable of copying