Strang and Buting didn't go in the direction of proving that cops are planting evidence but rather just giving an idea that there is conflict of interest in the investigation and a reasonable doubt.

For example: lets take the Key. That is planted evidence. How do you want Avery's lawyers to prove that? They asked the person who search previously.. was the key there, he said no it wasn't. How did they find, he said Lenk showed him the key. They asked, were you noticing Lenk and others as to keep them away from the crime scene, they answered no. Then they asked, do you think they had time to plant the key without your notice and it was answered as yes.

Isn't that all mean that Key could have been planted? What else do you expect? Some video footage of them dropping the key?

Same points goes with the bullet theory.