What is the meaning of: 消矢? No Japanese dictionary I use seems to have it (not even the slang zokugodict.com, nor name dictionaries), nor do Niconico or Pixiv's dictionaries. Google seems to reference a mostly broken references to similar structures like "初音ミクの消矢" (Hatsune Miku), but nothing solid.

It's not 「矢」, but rather「失」, as in「涼宮ハルヒの消失」. The Miku song that you found references the Haruhi title. It's probably just sloppy writing, I think.

Tagged appropriately.

That should probably be 消失. Minor typo on the artist's part.

Edit: hatsune_miku_no_shoushitsu_(vocaloid)