Rubbish

"And she drove off into the sunset 'heroine style' with the emboldening encouragement and character-affirmation from Sandra.

Yeah, right.

Sorry - all total BS - and please don't tell me my detection of 'sardonism' is deficient."

^What does this post even mean? It really sounds like you didn't get it. The point was that for a brief moment she considered change - she had what was a rare moment of actually seeing herself as the rest of the world does. She had an epiphany. She toyed with the idea of changing, but Sandra was basically a codependent enabler who gave Mavin just the little bit of doubt she needed to disregard her strong doubts about her way of being.

In the typical movie, that moment of clarity would've been led to a happy ending with a brighter future as Mavin saw the error of her ways and turned over a new leaf; but in reality, people have epiphanies and realize that they need to change all the time, but then slip back into their old patterns (if they even try to step out of them) because change is much more difficult than staying the same, familiarity feels safer than the unknown, and thus, people rarely change much when it comes to their personality flaws because most lack the motivation to face the challenge.

So we see that Mavin was convinced (by codependent Sandra) that it was better to stay the person she was and went back to believing she was superior to others despite her emotionally stunted, solitary life. It didn't take much to convince her as it was much easier to stay the same than to change which truly would have been hard work.

And for her part, Sandra played a huge role in keeping the cycle going for BOTH of them in the end. She pumped Mavin up as something special, and then Mavin disregarded her worth and casually rejected her (just as she did when they were in school together and Mavin forgot all of Sandra's help - and even her name).
I think you made the film-watcher mistake of accepting everything a character says or taking it at face-value. Sandra's speech isn't coming from a place of wisdom, it's basically her own pathology speaking to cheer on Mavin's pathology.

The point is that Mavin's a heroine in her own mind and nowhere else. She carries herself as if she is one, but those who have an outside perspective can see that she's anything but. It's a false confidence that she has in the end which will keep her stuck in the rut she was in at the beginning of the film. She's literally delusional.