I've attempted to show how the art could possibly work with examples of Cell have Ssj2 aura and sparks despite not being Ssj2, and Piccolo having Ssj aura despite not being Ssj and few other examples.
That's a different thing. No one is arguing that the SSj2 aura is specific to SSj2, but that all SSj2's in the manga have that specific aura. You could give Krillin an SSj2 aura and it wouldn't affect what state Gohan was in because all that would tell us is that you don't need SSj2 to have that aura. In Gohan's case though it's the LACK of the aura that makes it clear he's not SSj2.
In other words, you don't need SSj2 to have that aura, but you do need that aura to be SSj2.
It's kind of like how bad Launch has blonde hair, but she's not a Super Saiyan. However all Super Saiyans do have blonde hair.