Thank you for the additional information.
You're codesign -vvd command argument is incomplete. The path needs to be all the way to the Install Adobe Flash Player.app, in your screenshot, it only has /Volumes/Flash Player/ One way to do this is to drag and drop the file, Install Adobe Flash Player.app onto the Terminal window after entering codesign -vvd command.
The PPAPI plugin for Opera is flashplayer28_0_r0_161_macpep.dmg
Can you try installing the other more recent versions, from the Archive page? Are you able to install 29.0.0.140, which is the version prior to 29.0.0.171.
Also, another way to update would be to opt into background updates and when Flash Player checks for an update, it'll find one and installing it silently in the background. When installing, this is update option 'Allow Adobe to install updates automatically (recommended)'.