Mhh, some progress! o)
I can set the toolbar to be a different one than "Image Viewer" and then I can get single buttons of any other toolbar to show up at least by hovering the mouse in the upper section of the window. On window resize, the toolbar will be gone again until I restart painting buttons with the mouse cursor. I can get it to show completely by pressing "" and if I open an image by doubleclick (fullscreen viewer), I can see the toolbar flicker once very shortly if I'm lucky and then its away again. Getting parts of it back by hovering the mouse also works in fullscreen.
Unfortunately, all this does not work with the "Image Viewer" toolbar, it's prefers to hide completly.
EDIT: Jon you nailed it. CTRL-N only works for the "Image Viewer" toolbar it seems. I can indeed make it show up with that key-combo.
So now this is different. In case the image toolbar is off, but set to be not the "Image Viewer" toolbar, all the magic described above can happen.
I had a look on how to enable the image viewer toolbar by myself (in case there is an option somewhere), I couldn't find something, but now I learned about CTRL-N! Thanks! o)
Btw: I needed to check "Always use toolbars keys.." in the customize toolbar section to get at least some key-combos to work.
It seems these changed quite a bit from earlier days and are not visible anymore in the viewers context menu.