I feel like this scene is enough to justify Jenny not being Ms. Gilmour. There's no way she'll have that kind of mental breakdown in the woods about us hiding the truth and potentially leaving but then proceeds to take up an entirely new identity while her entire family and friends thinks she's missing; or worse. She would have to abandon us after one of the most emotionally intense goodbyes in the story so far, and she would have allowed her daughter to grow up without knowing her real identity.
I don't know, it just doesn't sit well with me, it would be hypocritical and a huge jab at Jenny's character to pull that stunt, it's just something I highly doubt she'd do. While a compelling theory, it just doesn't really add up in my opinion and doesn't seem like something that the Jenny we've bonded with so far would do.
Even though I agree with you in principle there is a gap of 10 years from that scene until the power plant incident and many events may have happen to justify Jenny going to the past 20 years to 1989 and then it is many years living a new identity (30 more and on those years some are overlapping 2 versions of her) until 2019 to enforce her resolve about the decision. So isn't that unplausible that it may happen, of course on the basis that this theory is true.
I wouldn't say it is hypocritical, life change us, events change us, a son/daughter change us and we grow and start seeing things differently with a different perspective than when "we" are 21 years old. You simply base your opinion on what she is in that moment in 1999 and it is normal that we do, but there is so many things to explain in the story.
I love a good mystery, and this one mixed with sci-fi time travel. Now the harder part is to explain how Jenny/Ms. Gilmour went from that point A (1999) to point B (2009-»1989) to point C (2019) in a more and less plausible manner and this being a time travel story always raise a bit more my "suspension of disbelieve" state than other genres, so doesn't have to be a perfect explanation because there isn't on paradoxes.
Again, all this with the premise that this theory is correct, that in my personal opinion I'm certain that is correct on a probability of 90%+. It is too many coincidences, even the age of Jackie Gilmour in 2019 "confirms" this theory.