Late to the party but I do believe that the SA could make a case for obstruction of justice with just the two things you mentioned...washing the pants and spraying the trunk. For what non obstructive reason would you remove the smell and evidence of death from the trunk and pants?
The pants were washed before KC came home and told them the child was gone. Along with the spraying of the trunk.
They are not going to do this to the grandparents. It is just not going to happen. They have lost enough. IMO