When I talk about Orochimaru's full arsenal of snakes, I don't just mean the large ones. Hebi Sasuke has access to multiple snakes he can use as nothing more than body shields, like
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Do keep in mind that he was able to summon the snake and the snake wrap around Sasuke in the time it took for Itachi's Kunai to reach Sasuke. Also keep in mind that they weren't that
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from each other, so this is a really good feat. So yes, the moment Hebi Sasuke sees MS Sasuke prep Amaterasu, he should be more than fast enough to summon a snake to take the amaterasu for him.
The MS Sasuke that you are arguing for (the one that fought bee) is the only MS Sasuke that was shown closing his eyes before firing Amaterasu. No other form of MS Sasuke needed to do that. This is the prep I am talking about.
So if Hebi Sasuke counters the Amaterasu, MS Sasuke is going to suffer the backlash of using Amaterasu and Hebi Sasuke will be on his ass. MS Sasuke won't have the chance to prep another Amaterasu once this goes close quarters,
cause the moment Hebi Sasuke sees MS Sasuke close his eye to prep it, he shoots a chidori spear at his head and its over. So if MS Sasuke can't use Amaterasu, this turns into a CM2 3T Sasuke vs 3T Sasuke.