That version is just going to raise a lot more questions, like how the Hell did they miss a gun, was it really a drop gun, etc., etc.
Concur, on both counts.
Any claim that the gun found was planted has two problems: the dead guy did the contortions to get his handcuffed hands in front of him;
and the dead guy kicked out the car window. Did the dead guy do things that would be additional felonies just for the fun of it? Or as precursors to something like ... using a gun that the arresting officers hadn't found. Not that conspiracy-theory-grade silliness is something paranoid racists would recognize and reject.
As for why the arresting officers didn't find the gun beforehand, that is a good question. Did they fail to search at all? Did they do a perfunctory search? Were they interrupted/distracted? Was the gun hidden somewhere the arresting officers didn't want to go?