As you probably know, electrical current and magnetic fields are close friends. In an iPhone there is a compass, however there are also lots of cables and in all of them, or at least in most of them passes a current. How come the current doesn't drive the compass away (and far) from the real north?
About point #3: Try plugging numbers into this online magnetic field calculator: http://hyperphysics.phy-astr.gsu.edu/hbase/magnetic/curloo.html#c2
The Earth's magnetic field in places where Humans can be found, at least the non-astronauts, is about 1/4 to 1/2 Gauss.
In the final analysis, yes, the magnetometer is influenced by nearby circuitry, but steps are taken to reduce it, and just how much accuracy do we expect? The disturbance is within specs, whatever those specs are exactly. I wouldn't use a cell phone for life-or-death purposes, navigating huge sea vessels, etc.
Additional reading: "Understanding Smart Phone Sensor Performance: Magnetometer" http://www.sensorplatforms.com/understanding-smart-phone-sensor-performance-magnetometer-2/