Obito couldnt use Amaterasu even if he had both eyes. His MS abilities are different from Sasuke/Itachi's. Kamui, both variations, are his MS abilities. Just like how Sasuke has enton instead of Tsukiyomi. I know it's been stated that ALL who unlock the MS gain the same 3 abilities in Amaterasu, Tsukuyomi, and Susanoo, but this has been disproven throughout the manga multiple times. Theoretically he could use susanoo if he reclaimed his other eye but who knows. How MS/EMS abilities is still pretty unclear other than what we learned originally. My theory is just that exceptional Uchiha unlock exceptional MS abilities. Sasuke has Enton, Itachi had super Tsukiyomi, Obito has Kamui, Shisui has KA and I guess you could say Madara has perfect susanoo (but exactly how that works is unclear as well). While non exceptional Uchiha would only unlock the basic versions of the original 3 MS ability if they managed to unlock MS at all. Amaterasu without Enton, Tsukiyomi but to a weaker extent, Susanoo without the perfection. Imo when the databooks essentially stated that all Uchiha with MS unlock the same abilities it was actually only supposed to refer to Itachi/Sasukes abilities. Seeing as their brothers that would probably be the reasoning for their having basically the same MS abilities (except for the trade off of Enton for Tsukiyomi). Until we see more, which we definately will since there are 3 MS/Rinnegan users on the battlefield, all we can do is make assumptions. These are my assumptions lol
