The magnetic field statistics does seem a little questionable, and a simple good search gave me the feel that maybe you might have referred something else was tapering off - the ozone layer, maybe? I would also imagine that Mars wouldn't be that bad in terms of radiation. Europa, however, may be a little different.
Just some sidenotes from the IAC viewing:
42 raptor engines with 7 gimbals, 20% thrust. I found it very amusing that it ended up 42; I'm sure it has a mathematical reason, but it felt like a reference to The Hitchhiker's Guide to the Galaxy. The deep throttle range seems that they can avoid the N1 problem, if the computer has to shut down one engine, it can lower the throttle on 3 engines on the opposite side, rather than shut an engine off completely. I'm not sure about this though.
If things go well, ~6 years being the first flight trip to Mars feels a bit rushed and optimistic, but I feel Elon has a way of getting unrealistic deadlines. Maybe he already operates on the Mars year, given it's 1.8 years per Earth year. Heart of Gold was also an incredible name, consistent with his naming schemes.
By the time I got to the questions part I was let down. I was hoping to get answers to: Dealing with zero-G upon landing, radiation shielding, power generation (are those solar arrays actually enough?!) Though I guess that's why these discussion threads here exist.
Also, can they steer by throttling the outside engines asymetrically?
At the very bottom of the booster, you can see 3 (or 4?) "slots" or "spikes" protruding outwards. Meanwhile, it looks like the bottom of the booster kind of "sinks" in to the launchpad when landing. So - does this mean no more landing legs on the first stage booster? With the shown design, the booster slides into the landing/launching pad, which also serves as a refueling interface. Interesting (and intelligent) design.
Last but not least that amazing South Park reference.