If Aria is A, is she even left-handed?

Yep and here are some more clues : mental illness runs in the Montgomery family, it is believed she has MPD(multiple personality disorder) because she is always looking in the mirror and her costumes are always related to people who have two different personalities; e.g. who she dressed up as on the halloween train. On the halloween train subject, when she wrote her name on the glass it was in -A's handwriting, also A wrote like this when she locked Spencer in her Sauna and wrote a message on the glass. Aria must have come back to 'save the day'.