Does it secure configure uplink/and downlink to public MQTT server for admin or any other private channel?
Yeah, I was wondering the same question
When you create the admin channel, you assign a private key. So if I understand your question, the answer is yes.
It would need a device with the channel (and the key) on it, that is connected toi MQTT, but in concept that device could upload messages from the admin channel.
Although you can configure each channel separately if sends to/receiuves from MQTT - so can just makes sure that any device that add your privae channel, not NOT allow upload to MQTT.