Having a single button seems like a major limitation but it could be mitigated by a simple 2- or 4-button bluetooth keypad that could be worn alongside the watch or on the other wrist. Is the watch able to sleep until it receives a notification from the keypad that a button has been pressed?

>> Is the watch able to sleep until it receives a notification from the keypad that a button has been pressed?

Yes. The watch can be put to sleep and set to wake up on a button press or bluetooth message.