Answer to your first question is

1999
The first Flathead (10 inches) was caught in the Lambertville section of the D & R Canal in July of 1999. (The above three Delaware River Flathead catfish undoubtedly originated from a reproducing population in Schuylkill River.

I've also heard that NJF&G or PAF&G inadvertantly years ago stocked flatheads in the Delaware that were mistaken as channels from a reliable source. But I think it's safe to say flatheads have been in the system for at minimum 20 years.

I can't definitively answer your question about population and growth potential going forward but would ask this question. If they haven't expanded or grown in population of any significance in the last 20 years, why would we believe they will going forward?

I completely agree with you about the upper stretches of the Big D and the opposition from the trout angling community. I actually agree if it was a threat to that fishery, something would have to be done and would have been done already. Fact is there's no indication I've come across it's even remotely posed a threat.

My reasoning why I believe they won't encroach in those areas in any discernible numbers and haven't so far is water temperature and habitat. Brodhead and Bushkill and Flatbrook on the NJ side all empty into the Delaware near or above the Gap and are all cold water tribs. It's why trout flourish there. That's not the water flatheads inhabit nor the habitat they seek. I've researched the Brodhead, Bushkill and Flatbrook and haven't come across one report of a flathead found in any of those systems. Why, they're loaded with trout but water temperature and habitat are not compatible. I believe those variables alone will greatly impede further upstream expansion and why southern big river systems are more prolific with flatheads than northern rivers. I believe nature will address the boundaries of expansion and different management measures could help better control the overall population while embracing a true trophy fishery that could add millions to state economies.

Like I've said multiple times on these two threads, labeling them invasive accomplishes nothing because it doesn't change angler behavior or address what the state considers to be a perceived problem.