Inside the seat back the airbag is held into the frame by three brass-coloured 10mm nuts, if you're that far in you should already be able to see them. You'll need a ¼" drive, plus a UJ to get at the bottom one. Then it's just a case of unclipping and removing the wiring from the seat base to be able to remove the airbag from the seat back entirely.
The reason I say to remove the airbag entirely is because, assuming you have the same style replacement seat i.e. SE not Sport, the easiest thing to do is to swap the entire seat back frame in, rather than drill off the lumbar mech from the old seat and rivet it to the frame of the new one. There are just four bolts holding the back frame onto the base frame.
You'll need the headrest tubes out in order to get the cover off the back entirely. Inside each tube is a tang which needs to be pushed outwards with a suitable tool inserted down the tube in order to release it.
Sorry but I can't help you with removing/refitting heater pads or getting the cushion off the base, I was never able to to that