Given all the appeals allowable, this is pretty much already the case. Hell, technicalities have resulted in perps who WERE guilty beyond ANY doubt getting off scott free. The system is already VERY HEAVILY weighted toward a finding of innocence. Sure, mistakes happen, and even under the rules you propose, they will STILL happen--but all we can do is the best we can. I would argue that it is already the case that letting GUILTY ones skate results in more overall deaths of innocents than what you propose.
For another, before I'd ever feel totally good about capital punishment again, we'd have to scrap our present adversarial system of courtroom procedures. In particular, there would have to be as much of a positive incentive for a DA to drop an investigation of an innocent person as there was for him to prosecute a guilty one, and a gigantic negative incentive to prosecute an innocent person. I'd favor a three strikes and you're out rule for DAs; three innocent verdicts within five yeas, and your career as a DA is over."
Uh, this doesn't make sense. You want the DA to drop investigations of "innocent" people but you want him fired if three such verdicst are given within five years?? Now, if you postulate three GUILTY verdicts OVERTURNED ON APPEAL, then what you propose might make some sense.
There has been a big freakout over the effects of DNA testing. They thought it would eliminate prime suspects in one or two percent of cases and it turned out to be more like 30 or 35 percent. Given the fact that a prime suspect in a felony usually goes to jail, that translates into some huge number of people sitting around in prisons for stuff they didn't do.
Again, if there's any question, you can maintain public safety by keeping the guy in prison until the question is resolved. You can't unhang somebody.