As long as Clarissa and the others didn't realize it, that was enough.
Actually, she didn't go to such lengths as Kael had imagined. She didn't add any poison or any additional ingredient to turn the Elixir into something bad. It was simply a fake.
Zora was sure that once they tested the potions, Clarissa would surely jump out and claim that the Elixir of Tranquility had no effect at all, declaring it to be fake and using that to ruin her reputation.
With their limited brains, that was probably the only move they could think of.
Since Clarissa wanted to call Elixir of Tranquility fake, Zora would simply give them a fake Elixir of Tranquility.
Let them dig their own pit and jump in.
As for who people would ultimately believe, she already had her answer prepared.
And Morgana's appearance at the auction? That was clearly Cassian's doing.
Studying the refining method of Elixir of Tranquility?
