Ordered TLV3691s. Breadboard version with a 2.5 V reference and about 300 mV hysteresis sits at 4.8 µA in power-down and wakes cleanly at 11.79 V on my bench supply. Daily heartbeat added, and the offline rule. Thanks Laura, and Keisha the WDT version is my fallback.
Still waiting for the gateway to reconnect.