Ad 1) https://github.com/mysensors/MySensors/blob/development/core/MyGatewayTransportMQTTClient.cpp#L145-L165
Ad 2) I don't think it's needed ^

I would ask another question. Does the system work properly when there is no connection to the MQTT server?

Yes, the system will continue to function normally, although I am concerned about the "delay" commands in the MySensors library.
I have to do some tests with ETH/MQTT version, but currently I have no time for that.