I think either the degree of expertise of witnesses had been mis-described to the court, or their evidence about Declan's body was called into doubt. Basically there was a cock-up in the presentation of the case that led to the original verdict being overturned. I imagine Kimberly Hainey is leading a miserable, ill, addicted, despised existence out of prison, and will probably not live a long life, so I can't feel any anger that she's not locked up.
Declan's case seems very like that of Hamzah Khan but also very different. Both were tragically neglected boys with absent dads and addicted mothers who died, and mummified, in their cots, surrounded by utter filth. But whereas in Hamzah's case, he had been forgotten entirely by the authorities, in Declan's lots of people were meant to be supervising his mother and himself, but didn't do so with enough concertedness or frequency. I wonder if this stems from a failure in the system, or from underfunding and understaffing?