The capacitor is non polarized but they often have 2 terminals on one side and only one on the other. If that's the case then it's obvious which way it goes. Hate to say it though, my bet rides on the magnetron.
Even if that thermistor is behaving it is one of those dreaded 'bullet nosed black epoxy' ones that so often fail in these GEs that I would replace it. Check it (and the other 2 too) and see if the end the wires go into it is black. If so, replace them as the seal will likely leak rendering them totally inaccurate.
Looking at the picture on this page: http://www.appliance...fs22sbsbss.html Part 732 has a screw near the top inthe middle (on the bump) and panel 336 needs to be removed. You also have to remove the icemaker and maybe the Left hand side rail(s). Very typical of a GE freezer bottom.