Donald's reaction is troubling, but people react to loss in different ways. While he could reasonably be interpreted as indifferent, he could also have compartmentalized his grief and held it back.
As far a motive is concerned, even if Donald was angry at his parents, would he be angry enough to have his 3 sisters killed as well?
The Martins were not wealthy, and Donald had little to gain by killing his parents. As an enlisted sailor, I don't think Donald would have the funds to hire a killer.
The pistol could be a red herring. If foul play occurred, perhaps Mr Martin had the mystery pistol in the car at some point. Donald may have given it to his father, with his father unaware that it was stolen. Perhaps Mr Martin tried to use it for self defense, and it was somehow seized from Mr Martin during a struggle.
Donald may also have had contacts with detectives that were very upsetting. He may have been accused outright of killing his family by detectives, and decided to stay away from the area. We don't know if he hired an attorney.
At this point, I think several scenarios are possible. I'm less apt to see Donald as a suspect, and tend to see the incident as an accident. Still, if there's evidence, it must be acknowledged. I hope the remains found yield a cause of death, and if they were shot, some evidence. The sheriff's office would have to work on the remains, though, and there's a seeming lack of interest.