Yes, the power button is a open connection so only when it is closed something will happen (power on/off, standby etc.).

The order does not matter since there is no positive or negative. Image it the following way: the negative line is already on the mainboard while the positive line is connected to the button with two ends. So by pressing the button you will close this line and something will happen. Of course in this case it does not matter how you connect it since this wire has no specific polarity.

You can connect them where you like: at the button, at the mainboard or somewhere in between It will always be the same when the connection is closed by the button or the relay.