Heating just helps speed things up but isn’t required. Yes the water may/will rust the inside and I don’t think cathotic protection will work since free electrons will move to the outside of the sphere. Smaller is better as the time to empty and surface area to volume will lead to less opportunity for rust.

Alternately use a core that won’t rust it, perhaps glacial acetic acid as I doubt it will rust iron and can be vaporized until it escapes.

Easy. Melt some iron. put it in a syringe of some sort and slowly push it out of the syringe. When the last “contact” is leftjust have a wire or a fast moving blade to cut off the link.

One way we can use this vacuum thing and stuff is by observing the property that a falling drop has a spherical shape. If I drop liquid iron from enough height so that the iron solidifies before reaching the ground, I can get hollow spheres. (Vacuum because otherwise the drops will take tear shape).

I’ve thought of 5 other methods to solve this problem. I guess I’ll post them in my blog when I get time. 😛