I may be incorrect here, but suspect the CPS gave the go ahead for prosecution because the charges that had clear evidence to support them carry low sentencing options, but it was always clear that whatever did or didn't happen to end that babies life, the pair of them then worked to ensure the cause of death would be difficult or impossible to be determined.
Most people already know that some causes of death will be difficult to ascertain with sufficient delay and exposure.
There are some very telling details against the claim she/they might have wanted a postmortem, in that they then apparently accidentally did almost everything possible that would ensure maximized decomposition the longer they evaded discovery.
If the jury could see that, then it lends weight to the likelihood that CM/MG knew that they were either responsible for or contributed to her death, rather than she was a victim of SIDS in coincidentally appalling circumstances.
If no prosecution had been brought over her actual death it would have sent a message that this was indeed how to get away with it.
They would have faced very low repercussions for the more provable charges and outrage that no more serious accusations had to be answered, given what was already in the public domain.