Have you installed the pepperflashplugin-nonfree package? If not, try to apt-get it and then run this command: sudo update-pepperflashplugin-nonfree --install. Then restart Opera.
If that doesn't work, try to install this package: flashplugin-installer, and then restart Opera.
I should also mention that your manual installation does not work because Opera only looks in these folders: https://forums.opera.com/topic/5890/where-does-your-distro-install-pepper-flash/9