Most of that is already there, just not as part of god mode.
Ignore explosions: use the "no explosion ragdoll" ability, available when you hit a high enough level (you can use "whatitmeanstome" to level up in a hurry)
kill with one punch: "giveapoca" gets you the apocafist, which does this nicely
no knockdown: this one is trickier. In SR2, you could just change the <hit_by_car> block in gameplay_constants.xtbl so that a car would have to be moving at twice the speed of light to ragdoll you, but that whole block seems to have gone missing in SR3.