somehow the cops just know from grainy 140p footage

was miraculously not shattered into pieces, which happens to all other 3d printed guns.

the very well-built gun has a particular reload quirk that was seen in the surveillance footage

the doubt about not disposing the gun is a fair point. i suppose he either wanted to seed doubt to the prosecution (as someone else claimed below), or just forgot to plan this part

would naturally spend a long period of time sitting in a public place

fair point, but i think he simply settled into routine. this is corroborated by him being "visibly shaken" and not−well-prepared to someone asking him about the murder

including the additional time it would take for the cops to respond and then arrive

he obviously did not know someone tipped him off

a random McDonald s worker

slight correction: a fellow customer told the worker. if the concern here is that he would hide his face to the worker, well he may have dropped his guard after going back to his seat