I'm trying to understand how a prosecutor having a conflict of interest would be detrimental to the defendant's case. They're not expected to be impartial (almost by definition!) and the only way I can see conflict of interest being a problem is when they're being lenient on the defendant. If they're being too harsh, it's up to the defense attorney and especially the judge to restore balance, but I would expect prosecutors to be as harsh as they can otherwise.
Must be something I'm missing...