Who has the power to pardon state crimes is determined by state constitutions. Georgia's constitution vests the pardon power in the Board of Pardons and Paroles. Georgia actually has one of the weaker governorships out there. The most powerful man in the State of Georgia isn't the governor, it's the speaker of the house.
You can pretty much bet that a DA of a major city is a piece of trash. I've never met one that wasn't. I have some personal experience with this woman though and have absolutely no respect for her. As such, she's exactly the kind of person I'd expect would be a DA.
I'd need to look more into the bills currently proposed on the issue to see if they'd require re-indictment. Even if they would I'm not sure that's something the legislature could do. There's a lot that I would need to look into there so instead I'll just wait and see how it plays out.
As for why I don't think he can be convicted, it's because of the reasonable doubt standard. I'll put in this disclaimer, I'm assuming this is all based on what is publicly known about that call. If there's additional evidence it could change things. But based on what is publicly known, I can't see there being a conviction.
You don't have to believe claims that Trump just wanted accurate results. You can believe that he was trying to influence the election. The question is whether there's an alternate explanation that supports reasonable doubt. If his lawyers show up and argue that he believed that there was rampant voter fraud and he was pleading with the Secretary of State to step in, investigate what was going on, and find enough of the fraud to make the result reflect the truth, it's a hard narrative to disprove.
You don't have to believe that explanation. You can think he's lying through his teeth. The question is whether that explanation creates a reasonable doubt. And it does. Unless they have more then there's a fair chance the judge tosses this on summary judgment.
It's just so hard to get a conviction with talk in general terms like the call was. If he had said something like "maybe we can get you a few boxes of ballots that you can say were found" then you have him. You have something specific. General terms can be explained away as Trump asking the SOS to do his job.