Thanks Misty -- that seems it in a nutshell. Why all three fail IMO:
1) Only relevant for 1st degree murder charge. Doesn't matter for the grandparents because the computer evidence shows premeditation. Even for Nathan, unless he was killed instantly (which the blood evidence shows was not the case) I would argue (a) Garland made a conscious decision that he had to kill the boy too, showing premeditation, and/or (b) Garland unlawfully confined him in that spare bedroom before killing him, raising the crime to 1st degree murder anyway.
2) So what. There is a ton on non-forensic evidence putting him in that house beyond a reasonable doubt, and plus there is a perfectly logical explanation for the lack of forensic evidence given the haz-mat suits (or whatever their called) found in Garland's possession at the farm. Under the defense theory, no one can ever be convicted of this crime because of the lack of forensic evidence. Sorry, it doesn't work that way. They didn't kill themselves.
3) What happened at the farm does prove to me that Garland caused the deaths. That is the only "reasonable" explanation for why he treated the bodies in such a terrifyingly demented and horrific way, and the defense provided zero evidence to suggest any other reasonable explanation. Put another way, even if all we had as evidence was what they found at the farm, and the house wasn't in play at all (let's say it burnt to the ground) there would still be plenty of evidence to convict Garland (mainly the truck, the computer and the DNA found at the farm).