So, after sometime of using Opera 25 what I can say is, that Opera 25 is running much better (faster, more stable, no website problems so far) than Opera 12. I think it could replace Opera 12 in the near future for me. But one big point is still preventing this. It's the sceond point of my missing function list in my start post.
With nearly fifty bookmarks the bookmark management of Opera 25 isn't really helpful. It's much of scrolling, searching and clicking everytime for every bookmark. The shortcut solution is much faster for me, shortcut in the address bar, enter and finish. I use some shortcuts longer than I use Opera, 2009 did I switch to Opera.
This is still a problem I haven't found a solution for. I tried some bookmark manager of the Opera Store, but I haven't found one with this function so far.