Not written here before but followed this case from the very beginning. Found WS some months ago and have been following this forum since.
I think you have some really interesting discussions going on right now, and it was not my plan to comment out of fear to break the rules, which are many and not all familiar to me, but here goes. Just a theory that was sparked on your discussions around a planned abduction gone wrong and MM accidentally died in the appartment. Sorry if this has already been discussed or suggested.
If this was a planned abduction, and one that you might even have tried the night before, but the child woke up and cried for her parentes, what would be the natural thing to do in your second attempt? Maybe drug/sedate the child or do other things to ensure that the child does not wake up. If you use something like chloroform, this can easily go wrong and you accidentally end up killing your victim. Maybe you do not even notice this before you reach your destination and discovers that the victim has passed.