So you basically have broken the ribbon cable from the keyboard to the logicboard of which power is on the same array?
The only way to power it up without using the powerbutton would be to take the macbook apart and find the PWR BUTTON | | on the logicboard and short these two tiny connections together and it will boot.
And then use a USB keyboard or something? You might think this is mental but as a "laptop" computer it is f*cked until you replace the keyboard anyway.