"De dirigible iz in flames, everybody'z dead, and I've lost my hat." That's the type of plan that's been shown here. So unless this Fate chick caused stupid pills to pop with Wanda's rations, why would she (Wanda) make things worse for the defenders than they needed to be? "I'm fated to survive this battle? I throw away my sword!"
There were faults in the plan, but I'm not sure we can lay the blame for them all at Wanda's feet just yet. There are a number of circumstances that might have led to these ends. For one, maybe Wanda contacted Stanley immediately after he received the hammer. She goes into it thinking "Ok, he's a lot stronger in direct combat than most, but we can handle that". Then, AFTER the plan is hatched, he figures out how to tame dragons. He wouldn't necessarily call Wanda to inform her of this development. Why would he? As far as he knows, she wants him to win, and he's planning on winning regardless. The dragons will just make it a more crushing victory. So he flies in on dragonback, levels FAQ, and all Wanda can say is "Oops?"
Also, we know from the so-called 'torture' scenes that Wanda has at least some affection for Jillian in the present storyline. Presumably that affection would have been even stronger when they were still at FAQ, mutually suffering under Banhammer's treatment of them. Maybe she arranged for Jillian to be away specifically for Jillian's protection? This becomes especially likely if she didn't know about Stanley's dragon-taming yet. Maybe she thought the rest of their forces could handle it. Again... Oops?