1) Pretty sure it means those with the lock are running v2.5 or above which adds Public Key encryption to direct messages and enhanced session key based security to remote admin messages.
2) yes...wide open to anyone on that channel. The Key Size should be Default and the Key should be AQ==
Check out https://ncmesh.net/ especially the join page for set up tips. The MQTT may or may not be something you add and/or change.