go to lowes and get deep wire mold boxes and GFI plugs. You cant go from two blade plugs to two blade and a round ground, if there is no ground, must use GFI plugs
you see to have line and load confussed the line is the hot wires coming into the box a black and a white normally and the load is the black and the white going down stream
In my humble opinion you have a neutral problem start at the switch then the light then when you do the plugs try this trick plug a radio in and make sure it is turned on and loud to a known dead plug now go around to each plug and bang on it with the palm of your hand, if the radio...
If there are no heating elements involved and there is only heat lamps, I see no need for a dedicated circuit. If heating elements are involved you will need a dedicated circuit, Check the manuel that came with the buffet. It will tell you what size circuit is needed if elements are invloved.