I am not sure if it is legal to even distribute a sample text that might have no DRM.
But you have pointed me into the right direction anyway: I tested the whole exploding function again but this time on a full and definitely DRM protected book.
And guess what, Calibre prompted me that exploding is not possible on a DRM protected e-book. Which makes sense of course.
Since my intention was to alter the e-book layout without breaking any copy protection this thread can be considered closed. Sigh.
And thanks for your help once again!