Hi Barry,
As for syncing process, verification that the sync is working is straightforward. Add or change some password on one device, wait a bit and check it on the other device. If you see the addition / change on the new device, its working. The code is for verification purposes only, so you know you are approving correct device (You might have multiple pending approvals)
As for the backup identity, yeah, we are fully aware that it sucks. Thats why there is new implementation that won't require old master password. With it, every time you add a new device you will receive a code via email that you just enter into new device and it will be instantly authorised. Avast Passwords for Mac and iOS already have it and its coming to windows as well, in version 18.6 if things fo well, 18.7 if they don't.
Now, to resolve your situation. You mentioned this is 2nd device you want to add to synchronisation, so if the first one is properly syncing (you see that the sync is on in synchronisation settings) then all you need to do is approve 2nd device from it (you will see in sync settings that there is a device waiting for approval). Also, the master password you have on the 1st device is most likely the master password the self-authorisation process wants. We do not force users to change master password periodically, so it still might be the one you set up when you started using passwords.
Dan