Here's what I got so far, though: Tabitha & Jade must've been there either when whatever event started the town, or they created the event, or something. This was who knows how many generations back. They seem to have been husband & wife. Since they (I'm guessing) "caused" it, it's also on them, in some kinda karmic universal way, to "fix" it. But because they couldn't, every time they die, their spirit is reincarnated in such a way that their new form still ends up back in the town...because it's still on them to fix it.
In a prior life, Tabitha was actually Miranda - Victor's mother. That's why Jade recognized the song, even though he didn't know it - he knew it from a previous life.
The song obviously directly ties into whatever spiritual element is in the town. Given the whole story with Fatima, the baby, etc, I'm wondering if the creatures weren't somehow locked in an alternate dimension or something. Some have escaped over the generations, and how Fatima "conceived" & gave birth is a version of that? I dunno. Then they (the creatures) can "escape" one being at a time. But I'm wondering if Jade playing the song didn't unlock some kinda portal or gateway or something, letting a bunch of them come through.
The Julie that ran up on Jim at the last second was a future version of Julie. They've shown glimpses of time travel being a thing, but basically confirmed it when her & Ethan were at the diner and she was telling him of what happened to her at the ruins. She wants to travel in time to try to fix things, and he was saying she can't do that...all she can really do is observe. My guess is that she tries to anyway, and ends up either fucking things up -or- realizing no matter what she does, even if the specifics change, the end result ends up the same.
Wouldn't surprise me if next season (or later?) we see that wasn't her first attempt at trying to save Jim / get him to run / etc. That's part of why she's so frantic - not only is her dad about to get killed, but she's hoping beyond everything that this time she can actually save him.
That's just me speculating, though.