<blockquotes>That assumes that Earth would have had an atmosphere with equal insulation as Mars. That seems like a long shot to me. I would expect early Mars to have had more atmosphere than Earth, since it is closer to the giant gas planets. Its accretion disk would probably have been cooler also, allowing more gas accumulation.</blockquotes>

Would you like to read an elementary planetary science text which explains the nebular theory of the accretion of planets, the retention of gas by planetary gravitation, and the evolution of atmospheres.

When you've done that, offer a convincing explanation of how a plane with 1/10 the mass of Earth, 38% of it's gravity and 43% of its insolation could have a denser atmosphere.

You're fighting several trends:-

1) The amount of gas a body attracts and retains is closely linked to its mass, not it's position. If this were not true, the Galilean satellites would all have thick atmospheres.

2) The nature of an atmosphere is determined by those gases that are non-condensible at the equilibrium temperature of the planet, (Earth, Venus, Titan)

3) The pressure of an atmosphere is directly determined by those gases that ARE condensible at the equilibrium temperature of the planet (Mars)

When you understand these effects well enough to propose a viable alternative, I'd welcome your ideas...