la première version, diffusée en avril 1972 et rediffusée en 1982, ne comportait que 5 épisodes

This is true. I doubled checked and the original version broadcast in Italy on Nazionale only had five episodes.

I checked their archives:

Le avventure di Pinocchio (1972/) di Carlo Collodi

Sceneggiatura: Luigi Comencini – Suso Cecchi D’Amico
Regia: Luigi Comencini
Cast: Nino Manfredi – Andrea Balestri – Gina Lollobrigida – Franco Franchi – Ciccio Ingrassia – Vittorio De Sica – Zoe Incrocci – Clara Colosimo – Carlo Bagno
Puntate: 5
Produzione: RAI – ORTF – Bavaria Film – Cinerat
Rete: Nazionale
Data: 08/04/1972

And confirmed the airing with a random newspaper. The episodes aired five consecutive Saturdays at 9PM:

https://archivio.unita.news/assets/main/1972/04/08/page_008.pdf
https://archivio.unita.news/assets/main/1972/04/15/page_008.pdf
https://archivio.unita.news/assets/main/1972/04/22/page_008.pdf
https://archivio.unita.news/assets/main/1972/04/29/page_008.pdf
https://archivio.unita.news/assets/main/1972/05/06/page_008.pdf

I couldn't confirm the original run time for the episodes, but the version currently streaming on their website has 5x60min episodes (about 304min total): https://www.raiplay.it/programmi/leavventuredipinocchio/episodi/stagione-i

Ces 2 versions doivent-elles être considérées comme 2 séries distinctes ou comme 2 versions alternatives d'une même série ?

The latter: two different versions of one miniseries. Unfortunately, TMDb wasn't built to support different episode cuts. So we currently cannot support the alternative French version with six episodes.