If you were slowing down light to make it green you'd need to start with ultra-violet light, not infrared.
The real physics is well documented on Wikipedia. I recommend reading their page on non-linear optics.
How it's going to finish is really straight-forward.
When the seven kingdoms have sufficiently weakened themselves via in-fighting, the Others will overwhelm the wall. Danerys lands on the shores of the seven kingdoms with her dragons. Jon Snow, who is clearly the prince-who-was-promised, joins forces with her to fight off the Others fulfilling the title of the series.
I also wonder if the polar shift will effect magma flows
... Unlikely; the fields are far to weak, and get even weaker during a field reversal.
actually, i'd expect this to be a likely effect. it's well known that the earth's magnetic field is due to the rotational motion of magma in the earth's core. It makes sense that if the magnetic field is changing, then the motion of the magma is also changing in some related way.
Whether this actually has an effect on the magma flows we see here on the surface... who knows. Maybe not, but i'd certainly believe it possible.
Always leave room to add an explanation if it doesn't work out.
