Couldn't the crashed both ahve been caused, and also caused the crashers to survive. If you generate an electric field, either negative or positive, you either get a dragging effect or a reflecting effect. If they first turned it positive, lets say the plane was negative, they would draw the plane to the island. Right before the crash they turned the magnet negative, so that it would repell the plane, making the crash softer.

It's possible, except for the fact that a plane is made of a lot of non-magnetic metals. (As evidenced by Sayid's metal that he was using to try and dig with under the hatch)

But I do think that the magnetic field that Desmond spoke of has something to do with bringing the people to the island ... if nothing else, it could have been what messed with the pilot's radar and radio and took him off course.