Channelling raiton chakra to extract the flames away is a plausible counter
what naruto did is basically releasing chakra out from his body to get rid of the flames,
raikages activate their shroud from theirr body as well, and once they do that, then replacing the raiton shroud or releasing it would extract the flames away, in a very similar fashion to how naruto did,
not seeing how its much different
Not gonna happen, RnY isn't a solid defense like Naruto's cloak, as shown when Amaterasu didn't even need to get past Ay's armor in order to hit his arm, the activation/deactivation lacks any kind of repulsive force to repel Amaterasu, so that isn't gonna work either, if it did, Ay wouldn't need to sacrifice his arm in order to get rid of Amaterasu - And if I recall correctly, the flames sticked to Ay's arm even after he deactivated RnY, he had to cut his arm to get rid of it.
He used his 9 tomoe sharinnegan to cast that amaterasu
Sharinnegan >>>>>>>> mangekyou sharingan
When has sasuke used larger sized amaterasu?
He used an MS technique in his left eye (the eye he uses to cast Amaterasu), "Sharinnegan" seems to be both Sharingan and Rinnegan in one eye, which could explain why he can use MS techniques separately with it, even if it isn't a good reason, Madara could use Susano'o with his Rinnegan activated. Not to mention that he merely used a small scale Amaterasu, nothing that MS Sasuke can't/never used, using your argument is the same as people implying that Madara's Perfect Susano'o was enhanced by his Rinnegan, or that he can't use it without it.
The poster above has already provided a scan for the bold.