Actually Obito's chakra should be stronger, he's in his prime where as Itachi never reached his, also Hashirama cells boost up stamina and power of jutsus which is why Obito isn't blind yet. Kakashi isn't a true Uchiha so when he used it he got the side effects much worst and that's when he was new at using Kamui but not when he was more adept at using it (unless he spammed it).
Kakashi never had Susanoo until Obito unlocked it after getting both of his eyes back then gave Kakashi his Sharingan to use; it was Rikudo chakra that allowed for PS to be used instead of just final Susanoo. Kakashi used Kamui many times, we know Itachi used it in the past but since he had his sight for about 10 years after awakening MS, it's safe to say he didn't use it much. Kakashi had his MS since age 12 or 13 and even if he doesn't use it, it still weakens his sight as Itachi told Sasuke about MS losing its light from the time you awaken it; using the MS jutsus only makes it happens faster as in Sasuke's case. Kakashi is older than Itachi so he had it for a longer time and we see him spam it throughout Shippuden.
Nope. Itachi is a descendant of Indra like all Uchihas but Indra's transmiguration went from Madara to Sasuke. Indra received the Sages eyes which granted him powerful chakra and Yin chakra while Ashura got the Sage's body which granted strong lifeforce (large chakra reserve) and Yang chakra. Sasuke got Indra chakra and Naruto has Ashura and before them it was Madara and Hashirama.
