Darts can easily die from untreated lacerations or skin abrasions.
Yes, this is also the case with darts. Parasite eggs/larvae that have been expelled through droppings will mature and look to re-enter a host, and the way that they usually do this is by burrowing through the skin and any organs that might be in the way. It's not always life threatening, but when the organisms by chance burrow through the brain to get to the lungs, GI tract, etc, it can cause obvious repercussions to the cognitive functions. I would have the other frogs tested immediately. I'm assuming you never quarantined these frogs? It's likely that the tank is now contaminated also, so they may require regular Panacur or dewormer treatments from now on.but some parasites can invade the brain and cause dementia in certain animals.