Update on this
I deleted the ldap accounts provider, uninstalled.
Reconfigured with different domain name.
All the users reappeared with different domain name.
I thought i'd had to re-create all of them
I have tried the offline messages and they seem to work now, strange, but i'm happy they do work
But the firstname.lastname@example.org user still isn't recognized as admin.
I will test another system (new install) if time permits to see if offline messages do work properly.
edit: now the second time i installed chat-server , i already had jabberadmins group created from the previous ldap
edit2: side question ; was the ldap supposed to retain all my users ?