Scenario 1, Obito most likely:
- Of course we can look at the fact that Itachi does have clones to use but there's also the fact that using things like Shadow clones(Explosive clones for example) requires a good amount of his chakra. Reasonable thing for him to use is crow clones which require the use of crows. However, these summonings
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have a limit
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which should mean the crows used to form that clone shouldn't last long in the battlefield leading to the eradication of those clones which are the main factors as they use less. If Obito plays smart, Itachi is left with the shadow clones and he can't use multiple of those or they won't even be able to use Susano'o which limits him to few of these and in the end prompts Itachi to save chakra as constant use of Susano'o eats a lot of chakra. All Obito needs to do is patiently wait for this moment.
- Obito can use hiding like a mole which makes things harder for Itachi as it's more effective to me. Kakashi pretty much fooled him with it but the main reason why the Sharingan won't be seeing Obito is based on the fact that he channels his chakra through the earth turning it into fine sand which means the Sharingan only sees Obito's chakra and not Obito since his chakra is everywhere. Obito can then grab
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and then even warp one easily rather than coming visible to warp which leaves him exposed as he'll grab them and easily warp before and intervention comes into play. He doesn't even need to play the going tangible and intangible game which leaves him exposed in both worlds.
- Kamui world can be a pain if you have 2 people attacking from both worlds but it's not hard even if he warps a clone. He just sets the flame formation(unless something stops him from being able to use this there which I don't know of) and trap them there from getting in his way while he goes to the real world. Plus what I stated regarding him not having to go tangible and intangible.
Eventually he catches the real Itachi either via outlasting or playing smart. Not to mention even if he doesn't use these strategies, he can just be in the real world and still pass through their attacks as he counters. They'll tire out from burning chakra leaving them opened for a backlash.
Scenario 2, LOL.