There's a pretty large body of evidence that Jenny is
Darci's grandmother, Jackie and that the accident
didn't kill her, but sent her back in time 20 years to 1989, where she got stuck.
I’m struggling to understand why you’re quoting me, as I’ve never supported or even mentioned this theory.

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.
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.
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.
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.
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.
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.