Whew! I think I got it...
The mistake was to use the includes of shared \build_dev when building in release.
There is only one little difference which made everything work: qconfig.h!
It defines QT_STATIC in the static \build_app1 ...
The debug config of the project created by the Qt Add-in has to be adjusted a little though:
Include directory $(QTDIR$)\include =>...\build_dev\qtbase\include.
Lib directory $(QTDIR$)\lib =>...\build_dev\qtbase\lib.
The project still uses \build_app1\qtbase as Qt Version.
That way, the Debug build looks for includes and libs in \build_dev while the Release build looks in \build_app1.
And the .lib symlinks are not necessary anymore, I just kept the bin directory symlink.
Just in case someone else wants to do something like this...