Safety Relay Coil resistance

The idea you describe can work, but it is very crude to remove the power from the coil by shorting the battery. At the very least you should insert a resistor to limit the current somewhat, as is you rely on the internal resistance of the battery, which can be OK for a 9V radio battery, but disastrous for 12V car battery.

The more common way to achieve roughly the same is to use a 'relay' with a Normally Closed (NC) contact, so you can open the circuit instead of shorting the battery. This is in fact how the common electro-mechanical doorbell works.

I assume you mean how long will it take the relay to open and interrupt power to its load. I'm not sure if that 300/minute spec holds the answer to the question, but it does suggest an upper limit of maybe 100 ms. In practice it depends quite a bit on what kind of coil suppression is used. For more info see: http://www.cbxelectronics.com/pdf/app-notes/app-note1.pdf