Hmm... that makes sense.
So in your most-plausible theory, if this story is simply something off-the-wall he desperately told to his mother in private to try to excuse himself in her eyes, does that mean you don't see SM as making a serious attempt to frame someone else?
I see theories running on a spectrum between (a) McD as the evil puppet-master, rubbing his hands together in diabolical glee as he carefully feeds his mother information and pulls the strings from behind bars to frame an innocent man and (b) McD as the impotent, spineless little puppet boy, controlled his entire life by his puppet-mistress mother until he finally snapped, now desperately spewing word vomit that's intended for her alone so she'll keep thinking he's a good boy - which she then latches onto, manipulating and publishing for her selfish purposes.
This view of the lawn mowing tale fits better near the (b) end of the spectrum. One of the most difficult things for me with this case is that, when I put on my guilty glasses, I can't make all of the known pieces fit well with theories on either end. The lawnmower tale is hard to fit near (a), unless (as I mentioned above) the underlying theme is, "This will be so purposefully crazy and stupid, nobody will believe it's a plan, and everyone will be so confused that I'll be off the hook". That's one reason I was trying to solicit ideas about possible scenarios the other day in which the saw package might have been intentionally left (because who could be that smart and yet that bungling at the same time?).
I'll need to ponder some more on (b), I think. Something about it doesn't sit right with me.