ive never found that the mig pushes the metal away, done numberous sills front radiator suport sections and the whole roof of the wagon with the mig.

You just need to make sure that the metal is clamped tight, as if there is any gap the mig will just fill it up and the 2 bits of metal will be held at that gap by the weld
When i did the roof i had a whole heap of g clamps holding the roof down adn then just used vice grips beside each hole i was welding. Would be better to get some of the actual metal working vice grips that have the 2 seperate fingers on each jaw so that they could clamp each side of the weld

i dont really see how the spot welder would work if its only welding one side of the panel...unless its just melting the one side till its heated up the other side enough for it to join. could achieve the same thing with a mig really. and you would still have clamping issues with it only being on one side

as an actual spot welder shoots the current through the 2 bits of metal, from one probe to the other, so that all the arc is focussed between the 2 probes...which is where your weld will be. they also clamp the bits of metal together at the same time

Without it having a probe on either side the arc will just act like a normal welder and not really try to join the 2 bits of metal together, as its easier for the electical current to travel through the steel to the earth rather than jumping from one bit of steel to the other and somehow trying to make a weld at the same time