Place(s) for ground (PE) and neutral (N) connections in TN-C-S
Where is/are the correct place(s) where PE (ground) and N (neutral)
  should be connected in a 230 V TN-C-S network? One or multiple times?


Only one single point. After that PE and N have to be separated all the way.


  is it correct (allowed, or maybe even necessary?) to have such
  additional connections between N and PE?


No at all.