Evoxius

Sure, but he isn't looking to micromanage their civilization, so if they can manage it themselves then that's for the best

Well, a place like the UK has a very high population density so how many people are still alive is a good question. And, how or why would Harry save people from the UK? He's in America, and America has a population of hundreds of millions of people. He'd have to decide to abandon all those people to fly across the North Atlantic Ocean for a chance to save less muggles that he doesn't know. There'd be little point in it. Harry isn't a muggle either, so it's not like he's nationalistic towards Muggle UK. Especially since he's grown up in both Magical Scotland and Magical France. If this were a variant of the Wizarding World, then yeah... he might do something differently. There's also a limit to how fast he can travel, so sticking to America is his best shot at saving the most lives. Maybe if he has more time, he can try expanding to nearby countries, but we'll have to see how that goes

Harry can't enter time-accelerated Textures with his Avatar/Will, he can at most observe it from Heaven or through the Hero's Journal. The Treasury itself is a void, so they wouldn't be able to visit there. But yeah, he can start learning Apparition. The only thing I'm still thinking about is the limitations. Do you need to have been to the location beforehand, or can you just make a guess with a good description or a picture... There's been a couple of Apparitions shown in canon where they hadn't been there before, but it is very overpowered as well... Little point in traveling anywhere if he can just grab someone who's been there, listen to them talk and suddenly he can teleport to someone's treasury, for example.

He's putting all his bets on the Church of the Creator right now. If they keep expanding at the same rate and converting people to become true believers, then he can get the World Power he needs. So... he'll probably have to put on more of a show