Top 5.... That's gonna be hard to do....
Each questions is necessary to me. I'm debating something.
You choose 5 and answer them. and ill choose 5 later. :itachi:
1. The Yata Mirror is endowed with every nature, yes, and it is true that it changes its composition to block any attack, but it is a common misconception that it
must change to the opposing nature, or to any nature at all to block.
Let's say a fireball is sent at it. Maybe it changes to water, maybe it changes to earth, maybe it changes to lead or diamond. Any of these work to negate the fireball, so all are on the table, as far as we know.
2. Yata Mirror's ability to identify an attack is based on one of two possible things, either
-Itachi chooses what is blocked and what is not, or
-The Yata mirror is sentient.
Both are possible. Both Itachi and Sasuke have shown that a Susano'o user can choose who enters Susuano'o and who does not, so the same may apply to the Yata Mirror, as it is seemingly attached to Susano'o.
Alternatively, The Yata mirror itself may select due to a latent sentience. We have seen that inanimate sentience is possible, as evidenced by Samehada, so being a tool of spiritual origins, tiers above the legendary tools used even by the Sage of Six Paths, that is another likely ability.
Re-reading, these two comments pretty much cover all of your (legitimate) questions.