I'm not sure why, but I haven't seen much written about the mirror move
of Spearow.  There have been interesting discussions recently regarding
the difference between attacks that directly deal with the opposing
pokemon, and attacks like Khangaskan's fetch.  Would Spearow's mirror
move be able to repeat a fetch, or is my first impulse right, that it's
like agility only that instead of avoiding damage and special effects
(poison, confusion, etc) it sends whatever was thrown at it back the
other way.

Mirror Move will copy the final result of the previous attack done to Spearow.  No matter what that attack did.  The only difference is that it comes from a colorless source.  That's it.

Scott