ELI5

You're right on most accounts. Imagine Mt. Everest compressed into a teaspoon and then suddenly released to re-expand all at once. That's a fair approximation of what would happen. It is going to expand as fast as the atmosphere will allow it, meaning it will superheat the atmosphere. It will likely result in all free water on the planet evaporating. The side of the Earth where it happens would likely be entirely molten. Given it will expand outward more or less equally, it would also likely eject some portion of the atmosphere into space. That also means there is likely a giant crater, but the molten surface will smooth that over in time - just like how when the Moon was formed from another large object smashing into young Earth we don't have a moon sized crater on the surface. None of this even considers the damage the global shockwave from the sudden expansion would cause. Mostly because everything will likely be gone already.

Why? Because the only thing making a neutron star a neutron star is its gravity. Take a portion of it away from that star, it will explosively expand. Remember, this is so heavy that protons and electrons have been ripped away from their atoms and smashed together to form neutrons.