When Does Odo Become a Changeling Again?

This episode takes place five months after "Body Parts". Miles O'Brien mentions not being able to witness Keiko give birth to Molly, an event which occurred in TNG: "Disaster". Odo regains his shapeshifting abilities at the end of the episode, having previously had them removed by the Founders in "Broken Link".

When did Odo harm another Changeling?

In the third season finale, "The Adversary", Odo killed another Changeling that was attempting to take control of the Defiant, leading to his punishment by the Great Link in this episode. In the first episode of the fifth season, "Apocalypse Rising", Sisko and his crew attempt to unmask Gowron as a Changeling.

Does Odo have a crush on Kira?

The fifth season episode "A Simple Investigation" seemed to confirm that Odo had indeed gotten over Kira, but a few episodes later, in "Children of Time", Kira herself discovers his feelings for her, and it is revealed that he is still madly in love with her.