Hi
After careful inspection everything is pretty Normal in the diff. The metal plate that holds the gear is still there obviously.
My mistake was due to improper knowledge of the gear retaining system, seized metal casing at first, with the missing retaining screw. When I tried to realign the metal casing in front of the screw hole , I had to put effort to move the metal casing which led in pulling it too much, then the gear was loose on its plate. The other mistake was made by trying to recover the gear using wrong tools (and in an impatient manner) , and we know how it ended.
I made the screw (see picture with dimensions and the original useless screw) and was able to carefully reinstall everything .
Metal casing is know properly tightened , the gear rotates freely.
The bottom line is that , as usual, I learnt a lot with this incident