Venus lacks a magnetic field, as does Mars. Mercury's field is less than one percent of Earth's. The gas giant all have powerful fields.
My understanding is that solar radiation interacts with the upper atmosphere of Venus to induce a magnetic field. So while it wouldn't protect structures in orbit, it would protect floating colonies and other similar structures.
But if a atmosphere (once established) were to take thousands of years to "blow away" then terraforming the moon could be worth it. And assuming you had a technological civilization for thousands of years, you could periodically replenish the atmosphere.
To me, it makes more sense to use pressurized domes or pressurize large lava tubes. Besides, I'm thinking most of the gas that escapes would end up on the Earth, so you'd have to consider how that would change the composition of the Earth's atmosphere.
Who says there are no nearby colonies? If there are resources to be mined, then people will settle there.
I would presume that would be part of the SAME colony. There are no other celestial bodies nearby that could support a significant colony. But I suppose you're right about Ceres being big enough for a significantly sized colony, and if you have a colony already there, it certainly makes sense to have a shipyard.
Look how the gold rush led to a proliferation of mining towns and major cities in the American West.
The gold rush didn't have automated mining systems. In fact, I'd be surprised if any of the mining was done directly by human beings. So the question would be whether it would be more cost effective to either build factories and processing facilities on Ceres, or ship out the raw ore to somewhere else.
However, you still need the colony first before you have a shipyard. Otherwise, nobody's going to work at a shipyard in the middle of nowhere like that unless it's black ops and they're either getting paid really well or they're criminals who would otherwise have been executed or served long sentences. (Which actually sounds like a great premise for a novel.)
The problem is that you've decided what you want the right answer to be and are arbitrarily rejecting anything that doesn't fit it.
I don't think that's the case. In fact, I've given explanations of how other places might be just as practical or better. For instance, I've pointed out that Uranus has abundant hydrogen reserves for fusion. In fact, I think I've implied that Earth or Mars are slightly better locations for shipyards.
I'm not so hot on gas giants because of intense radiation belts and inherent issues such as a much higher escape velocity than Earth. Also, they're outside of the Goldilocks Zone, so they get far less solar radiation.
So far, the biggest arguments I've seen against Venus are the acidity of the atmosphere and the difficulty in mining resources, both of which I feel are technical problems that could be solved, especially if the planet already has established colonies. Also, Venus has a slightly lower escape velocity in addition to it's mildly lower gravity, so escaping from atmosphere wouldn't be nearly as hard as launching for the Earth's surface.
Actually, Venus might be a good place to
hide a shipyard...