ok first, edit button, try it dont double post
second, didnt you ever think Gin and Kuchiki could be working together? every single time they were gathered and Gin was accused for something Kuchiki always looked with uncertainty. Also, not even GIN would be as stupid as to leave his sword inside Aizen for others to find...oh wait, might he have more than one sword?
I just realized, if the soul slayer size depends on you spirit energy, then Gin could easily take 2 soul slayers and input different spirit energy into them. Gin would keep 1 that everybody recognized and kill Aizen with the other one. (am i crazy?) Also even if the sword WAS the killer's only one, they cant really identify it because so many death god swords look alike....It'd have to be in the evolved form for the sword to be easily identified but it is not in the scene.