I think that the mathematics of the gravitation inside a hollow sphere would prove, not zero G as Newton estimated, but a straightforward two-body problem for any two points diametrically opposite. I'm going to have to spend a day some time trying to work that out.
I think that the Earth cannot help but grow like a geode. It would have been hollow inside in the first place. The original incandescent gases would not be attracted strongly to the center. A sphere would form with a hollow inside. More than one force would be at work. If the first hypothesis is true, the hollow sphere would attract material. Also, the outside will cool and condense faster than the inside. That makes the spherical shell denser, more massive, and its gravity will attract material from the inside.
The molten material in the shell forms crystals. Those crystals push at each other as they form and create hollow spaces. This makes them bulkier than the original melt. Then various forces bring more material in and melt it again, and form more hollows like lava tubes. Frothing gases make their contributions.
If there is a concentric sun, that's a big riddle. Would some type of small singularity tend to hover in the center rather than consume the planet? Would it manufacture elements such as iron, sulfur, uranium, and so on? Could such a singularity have been the "seed" of the planet inside a gas giant? Could a combination of its magnetic field and something like an "L0 point" keep it centered? I don't know how fine-tuned its emissions would have to be to keep the inner surface of the Earth at a constant temperature, but the water cycle works like a natural thermostat and moves tremendous amounts of heat energy.
You could also see a singularity as something that can gather material from the inside of a gas giant and then punch through the superconducting metallic hydrogen shell.
Hollow planets would be ideal for protecting life from supernovae in nearby star systems. I'm not too sure the Earth's surface didn't get waxed pretty good by a supernova a few thousand years ago. You can still see the iron trickling down through the soil along the highway, from clouds of iron that might have been traveling three percent of the speed of light. Did the Earth lose a significant part of its atmosphere a few thousand years ago while Mars lost pretty much all of theirs?