I understand that this is not a bug, I think we are expected to sort bookmarks into folders. Either you sort them, move your bookmarks from "Unsorted" to "My bookmarks" without sorting, or just take the extra click to go to Unsorted every single time you want to access your bookmarks.

Since I can't be bothered to sort 300 bookmarks or even move each one to My bookmarks when there is no "Select All" button, I hope there will be an option added to set Unsorted as default.

Looks like an OS issue to me; Opera starts in a windowed mode and the window edges simply go off screen. That's not something Opera controls, but your OS. When I start Opera (either normal or a private window) it always starts maximized, which makes sure that all the options show on screen. But as said: that's something I set up in Windows, not Opera.
You might want to try right clicking the icon which you use to start Opera, and then check the settings there.

I don't see why it would be an OS issue since it only happens with private windows. The window is maximized, but the buttens are not there. And also the panel is black when the window is first opened, it should have system colors, you can see it in the video.