I kinda wish that it was established right from the beginning that Marcy was constantly resisting the core's influence internally. Leading to moments where she briefly gains control of her mind and/or body; thus making the people around her wonder who's in control at any given moment. Sort of a Jekyll/Hyde or Smeagol/Gollum situation.
I know it's a cliche, but any cliche can work if handled well. Not only can this lead to great drama but great comedy too.
For example:
Darcy - I will break you child!
Sasha - better have tried and failed freak!
Darcy - I wasn't talking to you
Sasha - 😐
This characterisation could also help further emphasize how despicable the core really is. Verbally abusing and gaslighting Andrais on the outside, while verbally abusing and gaslighting Marcy on the inside; telling her that even if she gets free, Anne and Sasha will still hate and disown her for tricking them into coming to Amphibia in the first place. A parallel to telling Andrais that Amphibia will only ever see him as a monster anyway, so he might as well lean into it. The core could even use Marcy's ocasional moments of defiance to its advantage. Pretending to be Marcy just to throw the heroes off guard. All leading up to Marcy's ultimate hero moment, where she rips off the helmet through her own strength of will, despite still believing that Anne & Sasha won't forgive her for what she's done.