Kakashi doesn't know how to use intangible Kamui selectively since he's new to it and why would they need to Kamui only that part when many top tiers have attacks that would consume their entire body so they would either Kamui snipe the attack when it's in range or go full intangible. Also Kakashi doesn't have the chakra to Kamui large objects, he had Naruto's chakra to Kamui the 8 tails and he still left out a tail.
Yes intangible Kamui takes focus, all Doujutsus takes focus to use (Amaterasu, Susanoo, Kamui, kamui, KA,ect...). You have to focus on the target to use the jutsu and focus chakra into the eye to activate the jutsu. You need to focus on your enemy to know when they give you direct eye contact after you focus chakra (Itachi first closes his eye for this), KA is a selective attack so focus on who you want to use it on. Obito selecting which part of his body to Kamui or to Kamui the whole thing is being focus since he have to keep it active until he wants it off or to prevent reaching the limit.
How is going blind irrelevant here when that means their eye sight is poor and Kakashi stating he had to be very close to his target to even use it
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. No, only way to gain it's light is to get EMS and Madara didn't do anything to it but use it once to go get his eye, he wasn't planning on using it so had no reason on doing anything to it.
