Measuring almost 1 volt between Raspberry Pi input pin and ground, why?
The input in question is "floating" until the button is pressed, due to its inputs being of very high impedance. 

You need to add a pull-down resistor to the pushbutton, 10k is commonly used, thus:



That way, the Raspberry Pi input sees a false (0 Volts) until the button is pressed, then a true until released.