The panther cannot move onto any resource, even if it's a resource that you can't yet see (like copper, or horses, or coal). As far as I could tell when testing, this applies to all resources.
So we can use the panthers as prospectors. Any square they won't enter has something good!
Thanks for including the Adventurer save with this scenario. I wouldn't want to miss the opportunity to have Humbaba on my side, but immortal would ruin the experience for me.