The creatures are the OG people the Town that sacrificed the children with some entity to gain immortality, and were turned into aberrations, the souls of the children are probably cursed and can't find peace.
Even if one creature dies, they will be reborn, by impregnating a woman in town.
There is more than one entity on the town.
Reincarnations from OG townspeople are brought in to try to reverse the curse.
Jim died because he gave the clue that lead to the discovery of the song, and more than once he did things that angered the man in yellow.
The man in Yellow is the voice on the radio, and probably the voice on the phone.
Julie found a way to travel through time and from the looks of it. She was able to physically interact in it. If Jim would actually listen to a woman for once. He be alive, but he rather die being stubborn. Most man thing ever