on your first part, raikage broke those bones things which wasn't even sasuke's INCOMPLETE form, lol. if you remember sasuke activates his susano'o after the raikage fight, that is when sasuke boast about his "absolute defense" and then everyone proceeds to break it but couldn't. sasuke then breaks everything and leaves.but then again, susano'o isn't an absolute defense, as it was broken by 4th raikage several times. A lightning jutsu of kirin's scale and magnitude would most likely have broken through an incomplete susanoo. the yata mirror was shown so that the readers would know that with the sword of totsuka and yata mirror, that he becomes invincible.
on your second part, exactly, that is why the "yata mirror" was shown. the susano'o was shown to let readers know that the susano'o protected itachi from the blast, the yata mirror and susano'o both had their "shine" moment.
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