Why Legolas?

[Kath]

Legolas' somewhat neglected character

The character is neglected because the actor doesn't act! He doesn't stand there looking sad and stricken by grief at Boromir's death, he looks like he's trying to remember whether he turned his hair straighteners off!

As to why he was in the mirror, yes he probably was there just to put him in the film more. Or they may have filmed all the members of the Fellowship, but even in the EE didn't have time to put them in, or they just plain forgot. Or maybe it was to do with the inherent sadness of the Elves.