I expect we'll find out in the next update, of course, but a few possibilities:
- Nothing really changed; Jenny (and her as-yet-unknown daughter) still died in the accident
- In which case, if we're actually done with 1999, that really was a farewell to Jenny. I'll miss her, but she got a nice send-off (that was her reason--or excuse, anyway--to sleep with MC, in case they never saw each other again). You couldn't quite file this under "you can't change the past," because MC has (2019 Scarlett still has that picture, after all), but Jestur could channel his inner Doctor and say there are certain fixed points in time that can't be changed, or some such thing. You're always going to get wibbly-wobbly, timey-wimey in a time-travel story anyway.
- Jenny listened to MC, didn't work at the plant, and is alive and well in 2019
- I'd obviously be glad she made it. But would I feel the same way about 41-year-old 2019 Jenny as about 21-year-old 1999 Jenny? Would she feel the same way about MC? She's had a kid with someone else in that time, after all.
- Jenny listened to MC, didn't work at the plant, but died for some other reason before 2019
- Boo! Not as much because Jenny's dead as because it would really feel like a cop-out.
- Jenny listened to MC but was still affected by the plant accident in some way
- A theory discussed previously in this thread is that the accident, rather than killing her, sent her back 20 years (so to 1989), where she assumed the identity of Jackie Gilmour, making her Darci's grandmother. This of course introduces all sorts of complications, but goes a long way toward explaining Jackie's apparent attraction to MC. And IIRC her body was never found, so she was always "missing, presumed dead."
The first is pretty risky; killing off a popular character can alienate a lot of your audience, but it could be explained with the rest of the story, and the risk is somewhat mitigated by the fact that she was "dead" when the story began anyway. The third is even more risky IMO, as you're killing her off without even doing anything for the story. The second would be nice, but probably too easy.
I hadn't really considered your idea that Jenny might travel forward. At least one other 1999 girl--April (whom I wouldn't at all mind seeing more of)--has (if not all the way to 2019, at least far enough to see her nieces/Scarlett's daughters), so there's some precedent. Jenny's obviously very bright, so there'd be some reason for whatever shadowy forces are involved here to try to get her help. I kind of doubt we're going to play it that she's bright enough to have invented her own time machine, but I guess it's possible.