The idea that Mars had its atmosphere stripped by the solar wind is based on the idea that Mars had one in the first place. There’s probably no reason to think that the magnetosphere has any bearing on it at all.
That said, even if it did, and we introduced a 78 percent N 22 percent O atmosphere, presumably using blobs of frozen gases from the outer solar system, the solar wind would still require millions of years to get rid of it, leaving plenty of leeway for artificial replenishment.
Regarding Venus, it has a very slow and retrograde axial rotation. If that probably could be mitigated by a carefully planned large impact (assuming the planet has a lumpy distribution of density, as Earth does https://freerepublic.com/tag/potsdamgravitypotato/index?tab=articles ), perhaps using a huge chunk of ice, sufficient to cover the surface with water to a depth of tens of miles, changing the direction of rotation while changing the location of its axis.
After some long period of time, the planet would be cooled sufficiently to seed its ocean with aquatic plant life from Earth. Probably need GMO extremophiles to clean up the atmospheric residues first. A couple of thousand years should do it. :^)
Sometimes I toy with the (very) long-term idea of robotically mining Ganymede into lots of 20m cubes and using mass drivers to push them to impact on Mars to increase its mass. You’d have to do it robotically because the radiation from Jupiter would be quickly fatal.
Anywhere we can, we should. Even if only out of curiosities sake.