No one is saying maternity leave is evidence of unreliability, and if this was just she'd been off on maternity leave and they were terminating everyone would be up in arms.
You know full well this is not about her materiality leave. Please don't insult everyone's intelligence by suggesting we can't read and comprehend the ops posts and it's all about maternity leave.
The bottom line is the op is unable to do her role reliably due to personal circumstances. She doesn't even do it any more. I assume due to unreliability and the need for stability in the business, the work still needs doing, she is employed for a reason. Right now she does the bosses admin. Even when faced with dismissal she could not find a way to come in for a full day, this doesn't mean it's her fault, it simply is what it is.
The company has done nothing wrong here, their size is irrelevant. In five years she has worked approx 18 months, and due to age of children and year of for pnd,it would indicate much of that was in the first year, so over the last three years she's worked a few months max. On returning in September she's been available approx fifty percent of the time.
No one has anything but empathy for her, and it seems her employer does too, but most sensible people can see why the employer is now terminating and drawing a line under this.
Neither the employer or the op is at fault here, it's simply a set of personal circumstances that have led the op to not be able to do her role reliably.