I'd be surprised if Gerard thinks there's a serious possibility of Barnabas and Julia changing the events of 1970. After all, they didn't accomplish anything worthwhile in 1995. But even if Gerard believes they could change things, would it matter? I figure that if Barnabas and Julia change things in 1970, it will create an alternate timeline without eliminating the existing one. We know that at least one parallel timeline exists in the Dark Shadows universe due to different events occurring in the past. What we don't know is how a Dark Shadows timeline can be destroyed. Probably a silver bullet to the heart of the timeline would do the job, but how do you find the heart of a timeline?
The only way I can straighten this out is to look to science fiction concepts (because they work, they help us understand these ideas, and are consistent), at least for definitions of terms, and to the TV conventions of the times. Also, I'm not going to go beyond what they actually tell us onscreen, though it's fine to do that.
Anyway... the PT room apparently does two things: It takes you to a parallel reality occupying the same space at the same time as ours, and it takes you into OUR future, in THIS reality. While people clearly make different choices in PT, there has never been any indication onscreen that people's choices
created PT, and if anything was created, I think it was just the gateway between realities in that room, as someone's intentional project of some kind.
The "Many Worlds" idea of human choices actually
creating splinter-realities is not something that would have been in the minds of DS writers then, or anyone making alternate-reality episodes of any other program. The idea probably did not exist yet. It didn't make its way into TV till the '90s.
DS confuses things a bit by referring to PT 1970 as "another time" (to sound more romantic, even though it's the same time), and this can lead to both PT1970 and RT1995 being thought of as "alternate timelines" in some vaguely similar way. A "timeline" is the one and only way history has gone. The thing is, with time travel you can change that one and only way history has ever gone, so that that one single course of history is a different one "now"!
While PT is an alternate reality to ours, though, 1995 is
our future. When we see 1995, it is the one and only timeline. Unless one wants to introduce the very alien and (I think) disruptive, modern "Many Worlds" idea that the writers didn't have in mind, Gerard might think he can safely let Barnjulia go, but he's wrong. [spoiler]Once they (somehow) acheive all their objectives, the one and only timeline that exists changes, so that Gerard's ghost's 1995 never happened. Not only does Gerard not proceed into an alternate 1995, 1996, etc, still in control of Collinwood as a ghost, but the 1995 we saw has never happened, not in an alternate "timeline" or in any kind of reality.[/spoiler]
I'm sorry if you got all this already Lydia, and you just chose to disagree. If I'm belaboring all this, sorry. Anyone can choose to work "Many Worlds" into it, and I'm sure good fanfic has been written that way. I have a personal issue with it though... I dislike how it's reduced the possibilities for SF plots, because it's now practically taken as fact, just because a few physicists expressed a fondness for the idea a few years ago. But that's a tangent....