I think that the son and father were fighting and it got physical which explains the defensive wounds. I think Philip Fontaine then picked up the kitchen knife and cut Grant’s arm. Then I believe Grant disarmed the knife and stabbed Philip Fontaine causing his death. However I think that Ashley Fontaine came home to Philip’s dead body and tried protecting her son by trashing the house to look like a burglary and even went so far as to fire a couple of gun shots (failing the first one, hence the broken bullet) then called the police after getting her and Grant’s stories straight.
I'd like to find out whether the clothing they were wearing was confiscated as well, and if not, perhaps they should be retrieved and processed. I'm new to this site amd not really sure whether or not it's even possible to have this angle investigated further.