The single changing timeline seems to be the one the writers imagined, and they also imagined that when time traveler A is in one era and time traveler B in earlier era changes history, the world will transform around time traveler A via the ripple effect. BTTF writers Bob Gale and Robert Zemeckis wrote up an official BTTF FAQ where they talked about the related question of why old Biff returned to the same 2015 that Marty, Doc, and Jennifer were in, and they mentioned that 2015 transformed into 2015-A around M&D&J in the same way that 1985-A transformed back to regular 1985 around Jennifer on the porch:
1.9: When Doc and Marty are in 1955-A, Doc says they can't return to the future to stop Biff from stealing the DeLorean, because it would be the wrong future. But if that's true, how did Old Biff manage to get back to the same future that he left? Shouldn't he have come back to a different future?
A: As should be clear from the answer to the previous question, we believe Old Biff DID indeed return to a different future — a "2015-A," which would have transformed around Marty, Doc, Jennifer and Einstein (just as Doc explains how 1985-A would change into 1985 and instantly transform around Jennifer and Einstein). This would happen AFTER Old Biff returned with the DeLorean. For this reason, we made sure that Doc had caught Jennifer and exited the McFly Townhouse before Old Biff returned. Thus, by the time Marty and Doc are carrying Jennifer back to the DeLorean, there COULD be other residents in that townhouse — or perhaps the McFlys still live there. It is just as believable that the physicality of the neighborhood did NOT change as it is to believe that it did — so we didn't change it. We decided not to make anything of this idea because this is one of those difficult time travel concepts that general audiences have a real hard time understanding. (Try explaining this stuff to your mother and you'll see what we mean.) A detailed explanation of it would have slowed down the story, and most of the audience doesn't ever think about it. That's why we made certain things ambiguous and left various things open for interpretation in hopes that the possibility of at least one or two explanations would be better than a "definitive" explanation that you could find holes in. Let's face it, time travel is fantasy, so there's no way to "prove" anything. As filmmakers, we try to create a set of rule for our stories and stick by them, and stay consistent within the little "universe" that we've created.
For some speculations about what this implies about the "rules" of time travel in BTTF, see my answers here and here.
As for your comment However, Marty and Doc were never in the "regular" timeline to put her on that porch in the first place, one way I might think of this would be to imagine there was a security camera on the street aimed at Jennifer's porch the whole night; then in the final timeline at the end of part III, what would the tape show had happened that night? I think this is getting into issues Gale and Zemeckis never really considered, but if you go with the theory I suggested in the second of the two answers I linked above, different timelines appear in a sequence in "meta-time", and whenever a time traveler materializes on a given date, they always retain the memories of the "first" version of the timeline where they made that trip through time (see my comments in that answer about their memories and appearances on arrival being 'burned in' to all subsequent versions of the timeline). So, on Saturday Oct 26 1985 at 9 PM (the date and time they arrived in 1985-A in part II according to this chronology), there will always be a DeLorean appearing in the sky over Hill Valley, containing a version of Doc and Marty (and sleeping Jennifer) who remember having just had the adventures in 2015 that we saw in part II. This will even be true in the final timeline, where instead of returning to a horrific 1985-A they return to the "normal" 1985 we see at the end of part III.
I would also speculate that the "Self-Preservation Instinct of the Space-Time Continuum" which Gale and Zemeckis discuss in question 1.19 of the FAQ (and which I discuss in that earlier answer) causes Marty and Doc to still leave Jennifer on the porch in this different sequence where they return to a normal 1985 rather than 1985-A (since after all, when they left her there in 1985-A they hadn't yet realized the timeline had been altered), so that the security camera tape would show her ending up on the porch naturally and then waking up the next morning when Marty arrives in his Western outfit. But as an alternative possibility, I suppose the tape might just show her appearing on the porch via the ripple effect.
