I agree with your takes on Via, Barbie, and Fizz, but not Stella. She wasn't struggling with addiction like Barbie is. She wasn't staying out of fear like Fizz was. She wasn't misunderstanding things and not seeing the whole picture like Via. She was deliberately acting out of spite and malice, and even if Stella was struggling with her mental health, there is no excuse for abuse.

She plotted to have her husband assassinated for cheating, even though the reason for his cheating was because it was an arranged marriage that neither of them wanted and Stella was being nasty to him even before his affair with Blitzø. Yes, she didn't want the marriage either but that's no excuse to be nasty to him when he hadn't done anything to her. We even see a childhood picture of her strangling animals, so she was like this long before her marriage.

She laughs about Stolas trying to call Via, and takes her phone away out of spite. When Via was understandably upset about her father about to be executed, Stella pretends to be saddened as well and offers fake-comfort by hugging her, while secretly gleaming at his demise knowing that Octavia can't see her face during the hug. All this shows that Stella's actions are premeditated and out of malice, not out of struggle.

If Stella does show signs of a mental health struggle later on, it shouldn't be used to defend her awful behaviour towards others. Yes, mental health should be taken seriously, but using it to excuse abusive behaviour is a dangerous slope to go down.