Many thanks Harry,
I effectively totally messed up the pack loading, and understand now why I had so much issues (teared white tabs, etc) loading and using the back.
Thanks for the links, I forgot that Youtube can be a useful source of information, too.
My problem is now to salvage the rest of the Fuji pack, if possible. For now, I unloaded it from the back in the darkroom, and keep in a light tight bag.
Thanks again, best regards,