Can Reveillark's leaves-the-battlefield ability target itself?
For example, let's say its power and thoughness are turned to 0: it is destroyed (so its ability triggers) and its power is 2 or less (so it can be a target).
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Reveillark cannot target a Reveillark card in the graveyard, including itself. While it is in the graveyard, it has its printed characteristics, including 4 power. There are currently no cards that can change the power or toughness of a creature card in a graveyard. So, once it is in the graveyard, it is not a legal target of its own ability, even if it had power 2 or less while on the battlefield.
However, there is a way to get a very similar effect. If you have a Clone, for example, copying a Reveillark, and the Clone dies, it will have power 0 while in the graveyard, and it will be a legal target for the triggered ability. So it can bring itself back, and copy the Reveillark again, effectively achieving the same outcome that you are looking for.
No, you can't return Reveillark with its own ability. When it hits the graveyard, any effects that changed its power on the battlefield no longer apply. It is now a 4 power creature card so its ability can't target it.
400.7. An object that moves from one zone to another becomes a new object with no memory of, or relation to, its previous existence