What is the difference between papanca and sankhara, and in particular citta-sankhara? While contemplating both terms I could not find a difference between mental elaboration and mental fabrication. Aren't they one and the same?
yes it is not easy. First people  say that there are 2 triplets of sankaras : (kayasankhara, vacisankhara, mano sankhara) and (kayasankhara, vacisankhara, cittasankhara). they claim that they are not the same. See http://www.suttavinaya.com/wp-content/uploads/2018/07/Conditioned-Arising-of-Suffering-2018.pdf 
 Citta sankharas are vendana and sanna and they do not produce karma.
They claim that because they say that for instance, kaya sankhara  is the breath, but obviously the arhats still breath, therefore kayasankhara  still exists for the arhats.
Since they read that the arhats do not create karma, they infer that the kaya sankharas cannot be karmic.

The famous bikkhu analayo lists all those sankharas here https://www.buddhismuskunde.uni-hamburg.de/pdf/5-personen/analayo/encyclopedia-entries/sankhara.pdf

For papanca, people translate that as ''objectification'', like here 
https://www.accesstoinsight.org/tipitaka/an/an08/an08.030.than.html

https://www.accesstoinsight.org/search_results.html?cx=006639875531220445029%3A2z3mhfokk-u&amp;ie=UTF-8&amp;q=objectification&amp;sa=Search

some say ''proliferation''. It means thinking this https://www.accesstoinsight.org/tipitaka/sn/sn22/sn22.047.than.html
https://www.accesstoinsight.org/tipitaka/sn/sn22/sn22.055.than.html


  "He assumes feeling to be the self, or the self as possessing feeling,
  or feeling as in the self, or the self as in feeling.
  
  "He assumes perception to be the self, or the self as possessing
  perception, or perception as in the self, or the self as in
  perception.
  
  "He assumes (mental) fabrications to be the self, or the self as
  possessing fabrications, or fabrications as in the self, or the self
  as in fabrications.
  
  "He assumes consciousness to be the self, or the self as possessing
  consciousness, or consciousness as in the self, or the self as in
  consciousness.
  "Now, there is the intellect, there are ideas (mental qualities),
  there is the property of ignorance. To an uninstructed run-of-the-mill
  person, touched by experience born of the contact of ignorance, there
  occur (the thoughts): 'I am,' 'I am thus,' 'I shall be,' 'I shall not
  be,' 'I shall be possessed of form,' 'I shall be formless,' 'I shall
  be percipient (conscious),' 'I shall be non-percipient,' or 'I shall
  be neither percipient nor non-percipient.'