For information : I did 5 isos today, the 5th is being compressing file just now. I decided to get rid of the pager and removed it. So that the people who want it will configure it themselves once the system installed, therefore this problem does not exist anymore.
Now, lxpanel is started before openbox, from within a file named .xsession, which replaces .xinitrc : why is that ? That is because GDM does not see a .xinitrc, and he can see an .xsession, so I just renamed it, and that did the trick. (Simple test : try to start lxpanel from .xinitrc : you don't get a panel. Start it from .xsession, you get the panel).
pcmanfm is started from /home/guest/.config/openbox/autostart where one of the first lines call a shell script which is in /usr/local/bin...
I'll try the 5th iso in a moment.