Tbh I kinda understand the first story, Lily forgiving Heath doesn't feel that bad because they grew attached to each others before revealing what happened. Plus, Heath saved a child and was met with ungratefulness, which is what made her snap. Her story feels more relatable as the same humans she attacked are the ones who attacked her first, and she only responded on the heat of the moment. As a result, she feels responsible for the innocent persons who became collateral damages.
But here you have a mermaid stealing some random kid's voice by pure selfish interest, and after planning out her action. It's not on the heat of the moment, she premeditated her attack. And her only excuse is "I was an outcast, please understand". If you had to make an irl comparison it's like someone stealing your car and telling you "but I was poor", it's literally pure jealousy. Except instead of a car it's your child's voice.
They never met her before so you can't even say that they grew attached to her, but somehow it still all ends up with everyone being friend in less than a day. Her straight out malicious actions have absolutely no consequence for her and the situation is resolved in not even a full day with everyone holding hands with each others.
So yeah, in the end, my problem is not so much about "what else could she have done" since none of her actions could change anything anyway. It's more about "what would a mother realistically do in this situation". The whole story feels rushed and incredibly naive because of how it completely discards what would have been a natural human reaction to the situation and gives us a "let's all be friend!" resolution instead.
This side story is really badly written in my opinion.