Turning a high impedance output state to a concrete voltage value using a pull up resistor
Assuming the switch is a standard metal contact switch then yes D would be completely disconnected from S and can be considered high impedance as its impedance would be the air space inbetween the two contacts
Youre not 'turning high impedance into 12V'. This is because regardless of what you do to point D, so long as the switch is not ON, its still high impedance. All your doing is connecting a 12V source to a high impedance node such that current will not flow from the 12V source back to S. You would be correct in saying that you have a method for controlling a logic signal by changing the impedance seen at point D
That entirely depends on the logic chip. If the inputs of the chip are high impedance you would only need a small amount of current to drive it. A typical value would be something like 10k which would provide 1.2mA. You would need to read the datasheet of the logic chip to get a better idea of what requirements it has