Most things humans call facts are, in fact, theories. Very hard to think of exceptions, except for the Laws of Math and the Laws of Physics; Lyell's Principle and so forth.
Every single person posting here is, basically, theoretical - and yet, I do believe that all of you exist. Okay, maybe 1-2 of you are bots (but who the heck would do that? Make a WS bot?? Too much time on your hands, if you're a Bot-maker!)
Anyway, my speculation is that AM was on the property before the deaths, during the deaths and after the deaths. He somehow lost his clothing (seen earlier in the day) and did a lot of moving around when he ought to have been sleeping. He then left the property, taking MM's phone with him, tossing it en route to his family home (why he does that, I have no clue - not even speculation).
AM's guns were used in this crime. It was up close and personal. AM testifies that dogs and chickens were silent, indicating no stranger was around.
Hmmm. (That emoji is the MPOV, IMO emoji).