surely there must be some way he knows. But it's really speculation right now
He could have paid attention to Itachi and noticed Itatchi had some problems seeing or he could have heard/read about the effects somewhere. There was a scroll that detailed all of the information on the magenkyou, afterall.how would Kakashi know that unless he himself is also losing some eyesight in that one eye?
The Sharingan takes a lot of chakra to use when you're not an Uchiha. His eyesight isn't becoming worse. He just simply can't have it on all the time unless he want it to drain all of his chakra. Hence why he has it covered up.
It's not like only the Uchiha can read the scroll, they just had it because they were the only family who cared about it, since only they could use the sharingan. There could have been more information about it elsewhere in konoha, or maybe sasuke gave the scroll to kakashi or someone else who shared it.
Yes, but since Kakashi pretty much never uses genjutsu, using the magenkyou that way wouldn't really be helpful. We already know that there are multiple powers of the magenkyou, kakashi could just be using one of them that itatchi doesn't care about or like I've said in the past, the powers of the magenkyou might be partially dependent on your skills as a ninja.
