All of you make some good points here, but many of the fears are overblown, and will, unnecessarily, add decades to the space habitation timeline.
1) gravity - we know 0 g is bad, and 1+ G (up to at least 1.25G) works fine, but have little knowledge between those limits. However, even with long stays at 0 g, astronauts recover mostly when exposed to Earth's gravity, which would indicate that .35 g (Mars), with requirements to carry 50-100kg loads regularly (like Mars suits), will likely result in minimal issues on return. OTOH permanent stays may cause structural changes during childhood which could make a martian child coming to Earth's gravity a higher risk activity. Either way, having spin gravity on your spaceship increase slowly from Martian gravity to earth gravity on the way back is likely to mitigate most of that risk. Also, it is possible to place a centrifuge to generate earth gravity on mars - likely in primary fitness centers in each settlement. these centrifuges will also be needed on the moon, especially since the moon is only 3 days away, and the gradually adjusting the shipboard gravity isn't going to work on that time scale.
2) Radiation is less of an issue, although it does need to be accommodated. There are many small populations on earth which are exposed to higher radiation levels (2-4x) than allowed to nuclear workers which show no signs of extra radiation caused illnesses (near uranium mines or radon sources, at high altitude, etc.). The human body is able to dissolve radiation damaged tissue faster than the OSHA guidelines, which were generated using high dose, short duration experiments (20 years worth of space radiation in a few hour). At a certain dose rate, the bodies repair faculties are overwhelmed, while those same doses over longer periods are easily accommodated without harm. Iodine supplements and water wall shielding is likely enough to accommodate the flight time. - and the habitats should be covered in dirt to reduce lifetime exposure. Small children and pregnant women probably should not travel, due to the more sensitive flexible genetics of children, though, but they will be fine in the habitat.
3) repair - between robust, repairable design, redundant systems, spare electronic parts and 3d printers to fix any mechanical parts, repair shouldn't be an issue, except for permanently small settlements, where the expertise to do the repairs is lacking. Once you get to 400-500 people, you will likely have enough manufacturing ability to maintain your environment with only stockpiles of raw materials and sufficient miners to get most of the mass of those raw materials from mars.
4)All of these assume a reasonable mix of robots and humans. On the moon, robots only could be sufficient, since the time lag for communications is short enough, and replacements are only a few days to a few weeks away. this isn't true of Mars, which means Mars will need at least a small contingent of colonists to work the robots doing the preliminary work to set up the colony.
I fully expect to see a Martian colony of 20-30 people within 10-12 years. Musk is unlikely to meet his 2022 1st vehicle to Mars schedule, but seems to reach whatever goal he sets for himself eventually, so 2024 and 2026 cargo only precursor missions with a 2028 crewed mission is very likely. Many of those will remain on Mars, at least for a couple of years, until the next vehicle shows up. While the starship may be capable of carrying 100 people to a space station in flights of a few hours, it will need to provide additional room for months long trips to mars, so will only be able to carry, at most, 25-40 people. They will all be chosen, partially, due to their willingness to remain on Mars, so most will stay unless things are a lot worse than planned for.
Ultimately, I see this martian colony as the reason to put a village on the moon. those starships will need to be refueled in LEO, and it is cheaper (in terms of delta V) to bring oxygen from the moon (~2500m/s with aerobraking) than from earth's surface (~9500 m/s), and O2 (and the aluminum to store it in) is easy to find and extract in lunar regolith.