A lot of these issues were addressed very well in The Millennial Project, by Marshall T. Savage. That was in the ’90s, too. We know a lot more now.
One solution he had kills two birds with one stone: water is a great radiation shield as well as something we’d have to take with us to Mars. Instead of transporting water inside a ship, you transport it outside the ship to serve as a radiation shield. The outer lining of the receptacle can have a thin gold foil sheet that doesn’t weigh as much as getting lead or gold in enough quantity to serve as radiation shielding.
As a bonus, there are capsules being developed already that can decontaminate irradiated water. Perhaps there will be something light and easy to transport that will enable people to decontaminate water that has been irradiated during the course of the trip.
Another number that would need investigating is the efficiency with which water can be recycled onboard an interplanetary craft. So, it’s not totally straightforward to say a human needs at least half a gallon of water per day to survive. It’s less than that when you’re computing the total amount to bring.
I’m not saying Elon Musk has all the solutions, but a lot of people have been thinking about this stuff for a long time, and workable solutions do seem to be coming into view. I’m not sure rockets are even the best way to propel things into space, but that’s what Musk has been focusing on. I’m in the camp that sees Mars as a viable target for the 21st Century, but I’m also with you in thinking we really should start with the Moon first, because it’s a great intermediary launch point to move things away from Earth.
Your ethical questions are interesting. What do you see as the ethical ramifications of colonizing Mars, specifically? Let’s assume we continue to have no evidence of any life present on Mars at all prior to launching our first rocketload of colonists (and, by that time, we’ll have a lot more information about Mars than we do now).
Regarding going beyond Mars, you raise the following point:
What about the potential threat of other ecosystems moving backward towards Earth? Should all space travel be one-way to prevent possible incursion from alien organisms taking hold on Earth, with no means of biological restraint in place? Granted the threat of this would be very small, but it is always a potential possibility. With humanity seeking worlds like Earth, the threat of xeno-contagion rises if a planet shares more characteristics with Earth.
If we’re seriously making it to other Earthlike worlds, it’s a stretch to imagine we will have advanced so little in the biotechnological realm that we would have no means of biological restraint in the case of alien organisms. If those organisms are far more intelligent than us such that we cannot restrain them because of our inferior technology, it’s fair to assume they will find us first, before we find them.
Also, I don’t know how I feel about that term, “alien organisms.” You started off by reminding us that we’re tearing ourselves apart by thinking in terms of nation-states instead of holistically, and I would say about organisms on other planets that we’re all living in the same universe, so we should all work together and look out for each other, or face the inevitable Big Rip or heat death or proton decay that’s coming at The End alone.