Kakashi didn’t figure out he could use it until Naruto 689 he was an adult. He unlocked it and started using it around the garra rescue arc, and what like a year or so later they were fighting Madara and he admits he is almost blind in that eye so it would be tough to kamui Madara’s bijuu shield. Now let’s compare that with obito freaking spamming the crap out of it since he was like 13-14 years old.
naruto manga 689
When obito first unlocked it he literally used it like a 100 times against the mist ninjas. Umm yeah…. safe to say he would be blind by now. Sasuke unlocked his and was almost blind after less than a year of spamming it.We know that some uchiha may never unlock the sharingan, but the only reason they even have to potential to unlock the sharingan is because they are of the uchiha clan, I don’t understand why that idea is so impossible for you to comprehend.
Obito coming back and Kakashi using perfect susanoo. I think everyone is having an issue with Obito coming back is because everyone is assuming his spirit moved on to the impure world. I don’t think this is the case, If Obito moved on to the impure world with Rin why would she grab his hand and said lets go as if they needed to reach a destination?