Hi Loyer,
I predominately use the SBC for Raspberry Pi, and I find it relitively easy to find out whether the SBC is registered to 3CX - that being said I am not saying I would like visual recognition in the 3CX Management console interface (like you get with the bridge) There is an idea's page for this feature also which is "in progress":
https://www.3cx.com/community/threads/3cx-sbc-status-in-admin-gui-in-progress.42985/
Ways in which I would recognize that the SBC is registered currently would be:
Phones list in 3CX. I use Yealink phones also, a good way to tell if the SBC is up and running is the phones list (a supported phone will appear bold in this page with SBC: and the address in front of it).
You can also run your VERBOSE logs in 3CX and look at the 3CX Tunnel log, this would give you some ideas as to whether registration was successful or not.
In the Linux or Raspberry Pi versions you can also use the command "service 3cxsbc status" to check if the service is running on the SBC. However if you want further information then you can also tail the VEBOSE logs and detect for errors.
All that you would need is to enable VERBOSE logging on the SBC and then tail the log file and look for lines that contain: KA sent: X
Where X is an incrementing number that keeps going up by one while a registration is active.
If the registration drops, this is reset back to 1.
Also as far as your initial question about failing over between SBC and STUN I would have to say that although I have not tried this I do not think that this would work as a fail-over as you can either provision the phones via SBC or STUN separately, not both as standard - perhaps with a custom template it might be possible but I have not heard of anyone doing this.
Also you have the issue that if provisioned for STUN the MAC address is "hard coded" for 14 days by 3CX.
One thing I can tell you (as I tested it the other week for a customer) is that if you provision a 3CX Phone via STUN and want to move to SBC, if you remove the STUN extension from 3CX, reset the phone and connect via SBC you can provision it over via SBC without having to contact 3CX about removing the MAC.
Obviously this is manual intervention which would not suit your fail over scenario but useful to know.