>1) the heat trapped by the methane stays trapped even after it has decayed Not really. It would be present, but not trapped. Absent other factors, that would

>1) the heat trapped by the methane stays trapped even after it has decayed Not really. It would be present, but not trapped. Absent other factors, that would mean earth is above its equilibrium temperature and would cool once methane loss permits it to radiate heat more effectively. Either that or it shoots us in the face with a clathrate gun. There's really only one way to find out.