Ok, so this is all leading to one very specific place: An anarchic society of steampunk airships harvesting Deuterium from the shirtsleeve zone in Venus’ upper atmosphere.

Quite. Its just so.

That’s part of it but the dense CO2 atmosphere is the major issue. Don’t worry, we are trying to get there as fast as we can to Hell as well.

Venus's albedo is so high that the insolation at the surface is even less than Earth's. Yet it's hotter than Mercury, which is closer to the sun than Venus.

The article says that volcanism is the reason, and that solar heating would not cause this result on its own, even though it's everyone's first guess.

From TFA: "The sun alone cannot be responsible for making Venus the awful place it is today."

Venus is thought to have a (at least partially) molten core.

It doesn't have a magnetic field, but that could be due to the slow rotation.

Mars, interestingly, was just determined to have a core almost identical to Earth’s as I understand it. This is not the sole determinant of course - you still need enough volatiles, enough gravity to maintain a hold on the lightest elements across billions of years, and tectonics to keep refreshing the atmosphere. Unfortunately for us all, Mars has none of those. There may be other significant factors as well.

First order the explanation is simply, Venus is a hellhole because of atmospheric greenhouse effect exacerbated by proximity to the sun.

Given Venus's atmospheric pressure, I'm not sure that "no magnetic field protecting the atmosphere" is a big part of the story. It's got plenty of atmosphere left.

> no magnetic field protecting the atmosphere

Venus has too much atmosphere. That's the problem.