@hedge, you’re asking if we should terraform Mars if we haven’t already cleaned up this this planet. It’s a good question but I don’t see a problem here.
Let me borrow a quote from Isaac Arthur, youtuber and president of the National Space Society(in USA), and I’m paraphrasing him: If we have the technology to truly terraform Mars, then lot of that technology will already have been used to stabilize the climate on earth. It’s by orders of magnitude easier to “fix” Earth, than make Mars habitable to humans without the need for Domes, or spacesuits to breathe outside.
So to continue the “cleanup” analogy, it’s like cleaning up the worst nuclear disaster (Chernobyl ) vs cleaning a few drops of water off your kitchen floor.
No, not really. Mars has a very weak magnetosphere, so you need shielding against radiation. Also there is no atmosphere and any atmosphere we put on mars will quickly (in a geological timescale) get blown away by the charged solar winds because there is no magnetic field. So it’s an immense task, and probably a few hundred years out before we have the technology.