How to create a switch that when pressed opens, but after a time delay returns to the original state?
The standard method of dealing with alarm noise-makers is to have a "Silence" push-button. Pressing the button causes a circuit to latch and disable the noise-maker.

The circuit automatically returns to normal when the condition causing the alarm is cleared. This allows the noise-maker to be operational again when the next alarm occurs. 

It is rare that the silence duration is based on time rather than the source of the alarm signal being cleared but sometimes that is required. In those cases, the silence circuit is also (normally) reset to normal should the source of the alarm be cleared before the silence time period has elapsed.