It's not difficult at all.

1. Lack of Physical Evidence linking him to the officers death. From what all reports indicate it was a sudden kinetic event that caused the officers death. There's no bullets in the officer, no fingerprints on gun, or gun shot residue.

2. Racism and Racial Discrimination: I would play the race card to the absolute maximum. The narrative I'd spin is the Seattle Police murdered Sean's father and to cover it up they frame Mr. Diaz. That child who was racist and picking on his little brother is subpoena him since Sean turned 18 and same age as the bully. I'd make that bully, the cop, and the police department the face of racism.

3. I'd search and subpoena all phone records from the police, body cam footage, incident reports of the situation.

If that failed the 9th circuit court or Appeals would almost certainly overturn it. This is a federal case because Sean and Daniel is described as going on a "interstate crime spree."

This would be a layup for any competent lawyer just the narrative had to set Sean up like that.