I think Mel had to leave Toadie and the kids coz horrible Eden ( who I reckon must be a step son of hers) was threatening her and her new step family out of jealousy. So she left to protect them, and her talk in the letter of being a bad step mum was in relation to how she brought up Eden. Maybe she thinks she's the reason he became evil and didn't want the same to happen to Nell and Hugo or something. Maybe madam zelga told her something bad would happen if she stayed and she couldn't put them through it. Or maybe Eden was just threatening to hurt the fish family if she stayed with them.
Toadie will obvs realise he still loves her.
And then Paul will see how admitting the truth was the better option and can tell Therese he only left her also for her own good coz he loves her so much. Still haven't worked out what the reason can be for that yet, but it will definitely be something altruistic.
Although when they started talking about things feeling 'off' in the whole neighbourhood, I really did think it was going to have all been a dream....!