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
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.'
The word 'sankhara' has an extremely wide diversity of meanings. For example, it is used in 'ayu sankhara', which means 'life force' (MN 43); or 'sankhara khandha', which means mental 'aggregate of thinking' (SN 22.79). A Buddha or Arahant has ayu sankhara (DN 16) & sankhara aggregate (SN 22.85) therefore 'sankhara' here does not inherently refer to something defiled or unwholesome.
'Citta-sankhara' is perception & feeling (MN 44). It means that which conditions (sankhara) the mind (citta). For example, a pleasant feeling can condition the mind to have lust. An unpleasant feeling can condition the mind to have anger (MN 148). Thus 'citta-sankhara' means 'mind-conditioner'. Citta-sankhara is not sankhara khandha. Citta sankhara is vedana khandha (feeling aggregate) and sanna khandha (perception aggregate).
Papanca is uncontrolled defiled mental proliferation (MN 18). Papanca is always unwholesome. Papanca is the activity of a defiled sankhara khandha.
A Buddha does not produce 'papanca' because the sankhara aggregate of a Buddha is undefiled. A Buddha only deliberately generates pure thoughts, which are still 'sankhara' but are not 'papanca'.
You may refer this blog I wrote: The word ‘sankhara’ in context