I’m struggling to understand why you’re quoting me, as I’ve never supported or even mentioned this theory.

I quoted you because you were one of the people talking about Jenny's status and if telling her about time travel in 1999 changed anything about her "death", and my theory was explaining that I don't think she's even actually dead or that telling her would significantly change where/when she is yet.
What I mainly find problematic in your theory is the age issue between some characters, there are too many inconsistencies. It's difficult to believe Jenny could be as old as Jackie in 2019 without a solid explanation grounded in concrete details from the game. Without that, this theory seems unlikely, even if it’s an interesting one.
There’s also a problem with the timeline. Jenny is supposed to have a daughter before disappearing within ten years after 1999, yet Gabby, Jackie’s real daughter and Darci’s mother, is already present in 1999. This creates a contradiction. The idea that Jenny would take on a new identity, live a different life, and age unnaturally fast doesn’t hold up either, especially since Gabby appears to be roughly the same age as other characters from that period.
Clearly, Jenny couldn’t be her mother. Darci and MC even briefly meet Gabby when they rescue her from the lab at the power plant, where experiments were being done on her.
As for the timeline and ages, I already mentioned how time travel explains it just fine, as pitao further explains.
If we "draw" the Jenny timeline in that theory it would be something like this:
Born 1978 -----» 1999 meets MC (21 yo) -----» 2009 power plant accident (31 yo) and back to 1989 -----» and 30 years later in 2019 she would be 61 yo.
As for Gabby, if my speculation is correct and she
also time traveled from 2009 back to 1989 around the time of the power plant incident (remember, she goes missing shortly after the accident and is never found), her timeline would look something like this:
Born ~2000-2001 to Jenny (original timeline) -> 2009 Power Plant accident and back to 1989 (~8-9 years old) -> 1999 Abducted, missing, and rescued (~18-19 years old) -> 2001 Gabby gives birth to Darci (~20-21 years old) -> 2019 Present time (~38-39 years old)
So the timeline for her works fine too if we assume an un-intended time jump back from 2009 to 1989 related to the power plant accident.
Both Jenny and Gabby would have existed twice in the same timeline for a decade or two, but we've seen nothing to indicate that isn't possible (it's strongly speculated that the scarred guy is a version of MC and Darci has met an older version of herself, for example).
If Jenny were still alive, all we know is that she and her daughter are missing. Moreover, the theory that Jenny is pretending to be Jackie raises more questions. Why assume a new identity while leading a normal life without taking any precautions?
Why sever ties with Tiffany, especially given how close they are? Tiffany and Jenny are also sisters. That said, I’m playing with the multi-mod version that includes the I-patch, so details may vary depending on the version.
For the same reasons that anyone would need to assume a new identity if they were stuck 20 years in the past, mainly. Many of their friends wouldn't know who they were back in 1989, so they couldn't really go to them for help. And if they had reasons for letting events proceed in such a way that her first meeting with MC and other events were undisturbed, they may not even go to their family either. Getting in touch with a child Tiffany back in 1989 for example would also mean likely meeting Jenny's child self, and if she knew she
had to meet MC in 1999 without any outside influence, she might have avoided that. So it might very well be that her preparations specifically included setting herself up to have a new identity in a position where she could discretely watch over everything to make sure it goes as it should.
For all we know, now that MC has told Jenny about time travel in 1999, Jackie in 1999 might know that she can now make contact with Jenny without disrupting things, and might explain to her younger self why she still needs to go to the power plant and what happens, etc. That's speculation of course, but nothing would directly contradict something like this.
And finally, how does your theory explain the age gap between Jenny in 1999 and Jackie in 2019? Jackie seems much older than Jenny should be at that point.
The timeline pitao explained in further detail covers that.
On another note, I don’t see the point of using a time machine for something like this. It’s a huge risk, tampering with time and facing unpredictable consequences, all for a goal that seems unjustified. Additionally, this masked man suggested that making such radical changes is incredibly difficult, if not impossible, because time tends to restore itself to its original course, allowing only minor deviations.
I'm not proposing that she intentionally went back from 2009 to 1989 in the "original" timeline, but that it was an un-intended time-travel related to the accident. And it might very well be that the accident and her going back to 1989 is one if the events that is hard to change, so even knowing about it she might resign herself to it happening, particularly if it's revealed that things have to happen like this to avoid an even larger catastrophe.
To answer your question, I don’t think Tiffany ever reveals the name of Jenny’s daughter. As far as I remember, it’s never mentioned. We only know that both disappeared, and that chapter of Tiffany’s life remains painful for her, even years later.
I'll have to ask River where he got that name from, since he was the one that mentioned that in Discord.
In the end, while some elements in the game don’t completely rule out this theory, it’s hard to accept without a clearer explanation. We’ll likely need more details revealed later to make it more credible.
There's definitely unknowns still (namely, further details regarding Jackie's motives), but I think there's a lot fewer contradictions than it might seem at first glance once you do the age math.