Re: only question is...
"The central pressure of Ceres is about 133 MPa (19,000 psi). That exceeds the strength of typical rock."
Where did you get that value? Or, if you calculated it, would you please share your calculations? I've tried to do that before and didn't trust my approximations of the changing gravity from surface to core.
19,000psi is a bit high, but only modestly higher than the bottom of the Mariana Trench (15,000psi and change). It'd be challenging to have large chambers, but there are existing materials - like high-strength steels - that would tolerate the pressure well.
At a quick approximation, 9-inch thick Aermet 340 or Maraging 350 would give a safety factor of 2 on a long cylindrical chamber 4 yards in diameter. (You might want to apply a 10-20% strength knockdown when using steels that thick, though. They never get heat treated and worked as well as thinner sections.) Spherical chambers of the same diameter and thickness would half the hoop stress, but 12-foot spheres are a bit more cramped for proper Evil Layers at the Center of the mini-Earth.
Obviously, there are some practical challenges beyond pressure and high ping times. It's a bit of a challenge to dig to the center of Ceres (a record-setting tunnel that gets to very high pressures at its base) and work to excavate the core through that probably-narrow tunnel. Then there's the whole of challenges stemming from working on Ceres: near-zero G (centi-G, I guess), vacuum above, low temperatures, and you're probably a long way from the contractors you built your kit so service plans will be expensive. If your Caterpillar 2000X Asteroid Tunneler breaks down, you better hope spares were shipped with it.
One question I couldn't answer with a cursory Google search: what's the estimated temperature at Ceres' core? If it is much warmer than 100C, that's a whole new complication in terms of refrigerating the Evil Layer and maintaining material strength.
Another thought: the compressive strength of granite and basalt do exceed the pressures at Ceres' core, though their ranges are wide (100 - 250MPa and 100 - 300MPa, respectively). That's not a material you'd want for a thin-walled habitat at the center of Ceres, but it's enough that a thick, engineered shell of rock could take some of the load off a steel habitat.
"It's a bit difficult to hollow out a body that likely has a subsurface ocean."
Digging air-filled structures under water is an accomplished engineering feat. You - or at least I - wouldn't want to depend on asteroidal regolith and ice to hold a 14.7psi atmosphere, so the walls would need to be sealed in any case.
Biting the hand that feeds IT
