The reason the atmosphere doesn’t fly off into space is because of gravity, which isn’t caused by the metal in the core of the earth but by mass. The more massive something is the more gravitational pull it has. Keep in mind the sun technically has to solid core but the fact that it’s something like 90% of the solar systems total mass keeps everything in orbit. Also solar radiation isn’t that bad when you consider the sheer distance between earth and the sun, sure you’d get cooked if you went to mercury but the moon? Not so much, the temps only go up to a couple hundred degrees, and can be compensated for by cooling systems. As for the integrity of aluminum, yeah it’s pretty strong, everything from submarines to fighter jets have been made from the stuff, hell F-22s are partly plastic. Also the Apollo capsule was only pressurized up to an equivalent altitude of 20,000ft so the designers did actually have a concern but it was later found to be unfounded. I don’t know where you are getting the 5000 degrees from as I don’t think there is anywhere in either earth or lunar space where temps get that high.