I also have some weird sync issues.

First - the old devices.

For me it looks like this:

Yes. I found the thread https://forums.opera.com/topic/36848/opera-sync-remove-old-device/8 but I´m asking myself why its not possible to remove them via https://www.sync.opera.com/devices

Second - I have the same device multiple times. (I´m using three -> 2 Linux and one Windows)
Like this

The green lines shows my windows-device, the red my main linux-device and the blue my not so often used linux-laptop.

Why does this happens? How to avoid this?

If I manually delete the Local Storage\leveldb\xxx.ldb would they double/multiply again?
Why?

Third - I had to reset my Profile on one of the devices.
The password synchronisation synchronised an older password for at least one site.
I changed the password on the windows-device, recreated the profile and after syncing I had an old password.
Are there no timestamps?