I got the answer from our partner, saying that apparently, the locked options are by design.
It's a very frustrating situation, because :
-if we choose web meeting, then by default, everyone is an organizer, and it's clearly too many rights in the hands of everyone. We can switch the rights of everyone as soon as they get in, but it means that someone must monitor the conf all the time. It's complicated, as we don't always have someone to help the organizer with this task.
-if we choose webinar, then it's much easier, we can define the settings like we want, and make sure only the organizer gets in first. Great. But then, the invitation process is locked, and we can't use the Office365 option to invite people. It's very frustrating, as you need to invite people one by one. We have mailing lists to invite people by group in Office 365, but those groups are not synchronized in 3CX. And the sent email can't be modified as easily as with the other option.
Both choices are creating different problems, and it used to be just perfect with the web conference flexible settings. I can understand that there must be default settings, but to give everyone administrator rights is clearly dangerous. Is this going to stay the same ?