This is a theory I've kicked around a bit myself. Now I'm leaning toward mom not being actively involved though. I feel she may know or suspect some things but wasn't an active participant. However, filicide does happen though it is rare in a child Mac's age. Your scenario would be one of the few reasons I could think of for this kind of crime on a child Mac's age committed by a parent. Filicide is also much more commonly performed by mothers, and filicide of girls (as opposed to boy children) even more so statistically performed by the mother.
I do have some additional info that I cannot divulge the source of for fear of getting someone in trouble. It has to do with the probability that a knife set and something that looks like kitchen/butcher shears may have been used at some point. LE went to an area business with photos asking if any sets of this type had been sold there. Sorry, can't really say more than that. LE did NOT mention this case in any way, shape, or form. However, if you know Wenatchee, you'll know that violent crime here is a rarity, and crime with implements of that sort even rarer. I also haven't heard of any stabbings lately. And no- to anyone that is curious, they hadn't sold them to anyone they could recall lately. The business owner was flummoxed at the shears (they wouldn't sell them) and he said they hadn't sold that particular set of knives for a few years IIRC.
Ummm, Re the possibility of dismemberment, it's interesting that you would post that. Very early on I heard a *rumor* that one arm was partially or totally dismembered. Rumor only, mind you. I had thought that the supposedly brutalized condition of her body may have resulted from an unfinished attempt at disfigurement or dismemberment to make disposal easier.