I don't think T'Challa's perspective was really that bad. He didn't want to hand the cube over to his buddies, but in fairness he was able to sucessfully repell Hydra multiple times. So he had justifiable reason to believe the cube fragment was safer in Wakanda than with the other heroes. Both Stark and T'Challa ended up losing their fragments to Hydra, so in a way it didn't matter... aside from maybe T'Challa handing it over would have spared Wakanda an invasion.