And remember, a Kekkei Genkai can't be copied and Kakashi used to struggle with one Sharingan and he was able to use a goddamn Susanoo?! God, this is the biggest plothole in the shounen history.
1) Kekkei Genkai can be memorized and understood. The only reason sharingan users cannot replicate their effects is because KKG are physical, genetic features of a body rather than just a method of molding chakra like an ordinary nin or genjutsu, similar to how Sasuke couldn't handle using Lee's copied Taijutsu at first. Remember, prior to Part II, almost every shown and confirmed (Mokuton wasn't confirmed KKG, just special Shodai power) Kekkei Genkai was some kind of body mutation. Byakugan and Sharingan are eyes, Shikotsumyaku is the skeleton, Sakon and Ukon's unnamed KKG was a body thing, Hyouton being the sole exception. However, duplicating other Sharingan techs with the Sharingan makes perfect sense because you already have the base physical requirement to use the jutsu.
2) Previously, Kakashi only had the actual physical Sharingan eye with his chakra being put through it constantly, which is what caused the drain. Furthermore, Kakashi's chakra reserves boosted over the timeskip (recall: Kakashi can only use 4 Raikiris/day from full chakra prior to timeskip, but in Kakuzu fight he uses 5 in addition to a bevvy of other techniques) meaning that he won't run out of chakra from using sharingan. Also recall: in the Wave Country arc, a short use of sharingan totally incapacitated Kakashi for a few days. Even as soon as the Chuunin exams, we see him use it for protracted periods without losing mobility the day after. Kakashi's new sharingan, however, is not the physical eyes from Obito's body, but Obito's chakra turning Kakashi's eyes into natural sharingans. Because they are Kakashi's natural eyes, the strain on his body is reduced.
