Headscratchers / Star Trek Deep Space Nine S 03 E 26 The Adversary

• Why does the changeling change its shape when it attacks the crew in the final act? Wouldn't continuing to look like Odo be of tactical advantage? And when it changes its new shape, why is it an imitation of Odo's imperfect representation of humanoids that previous changelings took specifically to converse with Odo? What purpose does that shape hold in this moment?
• Why doesn't everyone just spit rather than use an elaborate blood test to spot the imposter?
  • Without close examination, 'spit' can be indistinguishable from water or other easily-attainable liquids. The changeling could just imbibe that and spit it out. Without getting into Alien Blood, blood looks like blood and is visibly blood at a glance.
• Why does Sisko trust Bashir to administer the blood tests? Sisko knows that he himself is not the imposter, and everyone else can trust him because he's bleeding. Wouldn't the best course of action be Sisko testing Bashir first, and then Bashir taking over?
  • Kira, the Bolian, and Eddington are being tested first because they are the ones under immediate suspicion because they all separated from each other while pursuing the Changeling. Bashir is under a lesser general suspicion because they don't know that he was replaced a while ago. They mention testing him as well at the end for the sake of thoroughness, but that's just to cover their bases and not because they actively suspect him.