Given that there are unethical prosecutors along with other characters in the chain of custody, I would prefer the state to corroborate cases in which the death penalty is sought with DNA evidence. Any malicious prosecutions, e.g. withholding of exculpatory evidense or evidence tampering, deserve the same penalty given to those unjustly convicted.
As you point out, DNA reports can be manipulated. One doesn't get certainty on DNA evidence alone. What would provide CERTAINTY of a capital crime? Some of those that would be convincing to me (off the top of my head):
1. Captured in the act (as with a cop killer.)
2. Multiple eyewitnesses otherwise unconnected to the accused.
3. Single eyewitness with multiple items of support.
4. Virtual eyewitness: video tape, audio tape, tv camera, etc.
5. Confessions in which the accused offers confirmatory evidence in addition to the confession.
6. Multiple points of biometric evidence that eliminate all other possibilities.