Sure, if we could overnight make all cars and trucks into battery EVs, replace all fossil fuel power plants with solar PV arrays (and wind farms too, since those are basically driven by solar energy too, though indirectly), and come up with some kind of storage technology to handle nighttime, high latitudes, clouds, still air, etc., then the energy aspect wouldn't be such a big deal. (Even better if we can make battery-EV airplanes.) But we're not there yet, and it's not likely we'll be there for quite a while unfortunately.
And even if we did accomplish that, that still doesn't solve the space problem. There just isn't enough room for everyone to have a big suburban house in a nice place with mild weather, and drive a personal vehicle everywhere. But if we tore down our car-based cities and rebuilt some just like Tokyo with that kind of density (and honestly, it really isn't as dense even as Manhattan; there's a lot more tall skyscrapers in NYC), then we actually could, while simultaneously having far, far fewer car-related deaths, and also having a much healthier population.
Wrong. Fusion reactors you can build at home are just novelties, and don't produce more power than they consume. No one has yet figured out how to actually do that, and until you can do that, fusion isn't useful as a power source. The only place we've ever seen fusion produce usable power is in a star. There's lots of research into building complex reactors to try to replicate this without needing something the size of a star, but no one's actually done it yet. It might be doable with enough funding, or it may never actually work; there's no way to say for sure.
The real problem is that too many people on this planet consume too many resources, and Americans are the worst. If most people lived in cities as dense as Tokyo and got around on trains and bicycles instead of cars, we wouldn't be having most of these problems. The amount of energy consumed per capita in America is simply unsustainable, and too many other people want to emulate this, but there's simply no way for that to possibly work on this planet: there isn't enough land or enough energy.