Answer to Question 1
O2 and N2 are poor absorbers of IR radiation and, thus, are not considered greenhouse gases. As O2 and N2 are totally symmetric molecules, electromagnetic waves pass by such molecules without being absorbed. Technically, they have no electric dipole moment with which the radiation can interact.
Answer to Question 2
The water vapor feedback is a positive feedback that tends to amplify small temperature perturbations. The ice albedo feedback is another positive feedback that tends to amplify changes to the Earth's surface temperature. Earth's climate is stable because of the negative feedback between the surface temperature and the flux of outgoing infrared radiation.
