1. The most immediate thing that would happen if that were the case is we'd already know. We could tell if the magnetic field were quickly changing. A lot of instruments people use every day would be affected by major changes in the magnetic field.
2. The magnetic field could completely disappear and we wouldn't get bombarded by radiation. The atmosphere itself protects us from radiation. Remember all that talk in the '90s about the ozone layer and how we'd get irradiated if it disappeared? That's in the atmosphere. The magnetic field doesn't do much for that AFAIK.
3. Based on the same evidence that leads us to believe the polarity switches, we believe it doesn't happen quickly. We are talking about a geological timescale here. The field won't flip by the end of the year any more than a new mountain will appear by then.
4. I think the biggest consequence if the field suddenly flipped is that we would have to fix a lot of navigational equipment.
5. You're more likely to get killed by the consequences of silly superstitions — say, based off a calendar ending on a particular date — than you are from a magnetic field polarity reversal.
In rough agreement with you, but have a quibble about your second point. As far as I understand it, while the lower atmosphere does provide significant protection, the ionosphere does have a substantial role to play as well.
[edit] Just thought and realised that the ionosphere is very little to do with the issue. I should really be talking about the magnetosphere and how it helps protect us from highly energetic particles in the solar wind. So my quibble still stands, but my reasoning was a big steaming pile of monkey poo.
Charged particles are the only thing affected by the earths magnetic field and they tend to be absorbed easily. Especially when they would need to get though 10+ miles of dense atmosphere to get to us.
Alpha Radiation can travel a few inches in the air.
Beta Radiation can travel a few feet in the air but far less than 50% can make it though 100 feet of air. So for 1 mile of atmosphere you would leave 1/2^52 of them.
But at ground level, cosmic radiation accounts for apparently an average of around 11% of ionising radiation exposure and this is measurably affected by the interaction of the solar wind with the magnetosphere according to experiment, or at least according to the nice graph on this page - http://www.sievert-system.org/WebMasters/en/contenu_rayonnem...
[edit] I think I remember reading somewhere that a strong magnetosphere also reduces the erosion of the atmosphere, but I suspect that this would not be a particularly important effect over the timespan of a magnetic flip, otherwise we would not be here to worry about it.
[edit] Have just gone and had a good look at this and it would appear that while there is an increase in cosmic rays at the earth's surface when the magnetic field is reduced, this basically has negligible effect when compared to other sources of ionising radiation exposure - http://www-istp.gsfc.nasa.gov/Education/FAQs7.html#q104 - so I was talking monkey poo all along. Oh well. Quibble revoked.
2. The magnetic field could completely disappear and we wouldn't get bombarded by radiation. The atmosphere itself protects us from radiation. Remember all that talk in the '90s about the ozone layer and how we'd get irradiated if it disappeared? That's in the atmosphere. The magnetic field doesn't do much for that AFAIK.
3. Based on the same evidence that leads us to believe the polarity switches, we believe it doesn't happen quickly. We are talking about a geological timescale here. The field won't flip by the end of the year any more than a new mountain will appear by then.
4. I think the biggest consequence if the field suddenly flipped is that we would have to fix a lot of navigational equipment.
5. You're more likely to get killed by the consequences of silly superstitions — say, based off a calendar ending on a particular date — than you are from a magnetic field polarity reversal.