It took a while, but I have now solved this problem so let me share the solution that I found. I did a combination of the following three things to made it work:
I needed a single cdUp() for the deployment situation as opposed to the 3x cdUp() which is necessary when running from Qt Creator (as it is explained here http://doc.qt.io/archives/qt-4.8/qt-tools-plugandpaint-example.html but on http://doc.qt.io/archives/qt-4.8/deployment-mac.html it says to use only 1x cdUp()) I was lead to believe that the argument for QDir::cd() would be case insensitive on MacOS based on the referred documentation. Instead, I now used 'PlugIns' instead of 'plugins' to use the standard directory in the app bundle to store my plugins in. I did so because using cd in a Terminal also required using 'PlugIns' instead of 'plugins'. On the other hand, when running from QtCreator, it should be 'plugins' as that is the subdirectory that QtCreator creates when building. So, here is a second difference between running from QtCreator and when deploying I not only applied install_name_tool to my executable to fix the library references but I also applied it to my plugin libraries. I made sure to use @executable_path references only to my libdfmcore and also to the Qt framework libraries (so, no @rpath references as generated by a build from QtCreator or by macdeployqt)If you wish to inspect my solution in more detail, you can have a look at https://modeltech.org/download/dfm/dfm.macos.zip