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This is the supposition: Venus is too hot, Mars is too cold. If they switched places then Solar energy would change to make both more Earth like. We might as well have had three instead of just one "Earth" in the Solar system, if the roulette of early planet formation had played out just a little bit different.

That's of course too simple. But wouldn't it help alot if it were the case from the beginning that Venus had had Mars' mass, and Mars had had Venus' mass? Or do other factors dominate mass and distance from the Sun?

Would Venus still have had a thick, but not too thick, atmosphere if it were at 1.5 AU, because it has mass enough to keep one, and wouldn't that have kept it warmer than Mars is today? Including flowing water on its surface under its atmospheric pressure?

Would a Mars at 0.7 AU have been warmer and maybe have had a passing atmosphere created from melting volatiles during a longer era in its history?

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You'll probably be better off by placing an "umbrella" in the Lagrange point between Venus and Sun. If it's big enough, the CO2 may even freeze completely - after a while. – Florin Andrei Jul 13 at 19:49

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This is a very interesting question. Of course, as you noted, you have simplified things quite a bit; there are other factors besides temperature that affect habitability.

Regarding Venus, you probably know that Venus is extremely hot at its surface not just because it is closer to the Sun, but because it has a thick CO2 atmosphere and is warmed by the greenhouse effect. There are, in fact, two things about Venus that would prevent it from being habitable wherever you put it. One is the lack of a magnetosphere, which is necessary to prevent ionizing radiation (particularly from the solar wind) from reaching the planet's surface. The lack of a magnetosphere appears to be due to the lack of a geodynamo on Venus, which has to do with the structure of its core. Second, Venus appears to lack tectonic plates, which you may know are responsible for earthquakes here on Earth. Interestingly, tectonic plates play a major role in controlling the buildup of CO2 in the atmosphere (see here for details). Lacking tectonic plates, Venus is doomed to have a large CO2 atmosphere wherever you put it, which would not make it a nice place to live.

Mars, on the other hand, is a very different matter. It has both a magnetosphere (albeit it is very weak) and it likely has tectonic plates (although last I heard it is thought to only have two). One of the reasons that NASA has sent so many probes to Mars is that it was thought to be habitable at one point. It is thought that Mars's growth was stunted because of gravitation effects from Jupiter and Saturn. So, in another universe, Mars could have ended up very much like another Earth.

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Just wondering about something, you mention that no matter where Venus is it will be 'doomed to have a large CO2 atmosphere'... Now my teeny brain wants to say, "That's okay because plants 'breathe' CO2 and emit Oxygen in return." So if indeed the planet were overwhelmed with CO2, wouldn't/couldn't that be acceptable for at least providing an environment for plants to thrive... and they could change the atmosphere for animals by proxy? – MegaMark 18 hours ago

Simply "swapping" position would not in and of itself be cause for a habitable planet. The key is "changing" position. If Venus was to undergo orbital migration it could conceivably move into Earths current position and cool down, continue to develop a more stable atmosphere, and produce oceans. Then further orbital migration could cause the atmosphere to collapse and marine succession. The question then is would it become like Ceres and Jupiter. Simulating a vector reversal, where Mars undergoes orbital migration and moves into the position currently taken by Earth, would warming it up cause it to further develop an atmosphere, and a marine environment. Then, when its vector moves it to the spot currently taken by Venus would marine succession and atmospheric reduction appear as Venus is to day. From physical appearance alone I would say there is a natural progression where the planets evolve as the orbital migration causes their vector to move away from the Sun.

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If, Venus had started out at Mars' position, it is possible it would've ended up evolving into an ice giant. At the greater distance of Mars the Sun's solar wind is weaker and would've stripped off much less of Venus' atmosphere, even in comparison to the amount Earth lost, as Earth is considerably closer to the Sun than Mars. So, Venus would perhaps have been left with a thick helium envelope and become a mini-Neptune/super-Earth rather than a second Earth. And also, if Mars had evolved at the distance of Venus, closer in, it would've been stripped of any appreciable atmosphere and possibly would've evolved into another Mercury, a grey and airless world.

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Doesn't the effect of solar wind on the atmosphere go the other way too? CO2 is partly frozen on Mars. Would a substantial part of Venus' atmosphere freeze to ice if it was as far out as Mars? Would ice on Mars melt to form atmosphere if it was where Venus is? Is/was there really any substantial amounts of helium available within the asteroid belt? – LocalFluff Jul 14 at 1:34

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