Let me try and answer so of this
Yes
Yes (credentials are stored on the devices that will allow the resumption of the mesh)
AFAIK - Not at this point
This thread has lots if interesting info about testing with large meshes - a common observation is that in a large (>10) mesh the end nodes (xenon in your case) should be sleepy (connect infrequently and send data and disconnect.) this is so that the total mesh bandwidth does not get congested.
This https://docs.particle.io/reference/device-os/firmware/xenon/#bluetooth-le-ble- has the best information re BLE - limits are listed at the end of the section linked above.