The rules set is full of exceptions. A thing is this until I acquire some feat or ability or situation or set of circumstances that makes what's normally this now treated as that.
I cannot find anything that suggests "treating" a weapon as light means it is or isn't light. Is this weapon still one-handed? Or is it now "light," and ineligible to benefit from power attack?
When "treat as" or "treated as" is used, it generally means for all intents and purposes, unless it specifies a specific set of circumstances for which it is "treated as".
For example, under the ranger's combat style ability,
If the ranger selects archery, he is treated as having the Rapid Shot feat, even if he does not have the normal prerequisites for that feat.
As there are no circumstances listed, effectively the ranger now has the Rapid Shot feat.
Also, when armor is created from mithral,
Most mithral armors are one category lighter than normal for purposes of movement and other limitations. Heavy armors are treated as medium, and medium armors are treated as light, but light armors are still treated as light.
Heavy and medium mithral armor are treated as one category lighter than normal for purposes of movement and other limitations. "Other limitations" is pretty broad though.
However, if you look at the monk entry,
Her unarmed attacks are treated as magic weapons for the purpose of dealing damage to creatures with damage reduction.
In this case, there are specific circumstances listed. Otherwise, they are not treated as magic weapons. They aren't detectable as magic, nor do they use an item slot, for example.
In the case of graceful edge,
If you do not wield a shield or weapon in your off-hand, you treat your chosen weapon as a light weapon.
To avoid confusion, I'm going to avoid the fact that off-hand only applies to two-weapon fighting and assume this just means you're a bipdeal creature and "your hand not armed with your chosen weapon".
In this case, you would not be able to use Power Attack any time you do not wield a shield or weapon in your off-hand, because you treat your chosen weapon as light, gaining the Weapon Finesse benefit. However, you could wield your chosen weapon with two hands, or pick up a shield or other weapon in your other hand and it would revert to being a one-handed slashing weapon, qualifying for Power Attack, but not Weapon Finesse.
As worded, you cannot use the two together (Weapon Finesse and Power Attack via Graceful Edge), other than with DM fiat. Having said that, this DM would probably allow it.