You have a guy with cuts and bruises, DNA evidence on him and the victim and he literally has the murder weapon in his pocket. Why would they go the extra mile to check anything else? They thought they had him dead to rights. The only reason they went after a confession was to expedite the process but they went in full steam ahead. I think you're giving police too much credit if you believe they would continue investigating beyond that.

Cell records aren't going to trump DNA (blood and semen) and a murder weapon.

I think it makes perfect sense that they stopped right there...Box going back and reviewing it all says more about the type of person/cop he is...his gut told him something wasn't right so he kept going. If another cop was on the case they never would've done that...hence the one guy continually asking him "you're still here? go home...hit the links"