With the agreement with Julia and Beth done, Arabella returned to the mansion and reported what transpired to her husband. He was waiting by the door ever since he sensed them returning.
After it was verified that Lucille was the plotter and main culprit for what happened, Ferdinand wanted to catch Lucille immediately.
But Ferdinand also held back since King Neville was still the Head of the Council and the current representative of Medeus. He decided to wait since King Neville would step down once this Assembly was over.
After all, even if they get Lucille now, they wouldn't be able to punish her as they wanted once King Neville pleads for her.
Thus, they had to wait until the Assembly was over.
Further, if Lucille was related to the production and distribution of the drugs added to Andrew's drink, even her father would not be able to protect her.
Arabella and Ferdinand then focused on having the drug investigated first and observing Lucille to see her next steps.
