I would just try it without the steam trample thing. It couldn't hurt. It's better than doing the same thing over and over and hoping it turns out differently (although it could).
You shouldn't really need to go building-to-building more than 3 times. Spawn side -> Power side (turn it on) -> Spawn side (buy Quick Revive & Speed Cola, down yourself, buy Who's Who) -> Power side (loop zombies on roof and then PaP + buy last 3 perks).
Let us know if you finally get it and what the key was.