WHM is stuck with Fluid Aura because they specifically learn "a water spell" during the Lv15 quests (after focusing on earth and wind in the previous two quests). They'd need to add something else instead if they removed it. Keeping it as a lower-power instant cast spell equivalent to BLM's Scathe might work.

And they're not going to remove the cards from AST - whether they want to or not. It's woven throughout the storyline, it's a large visual part of their weapon, and I'm not sure what would even be left of the job if you removed cards and Nocturnal Sect.