But the way you describe it, it seems to take for granted that there is only one time dimension. I think very existence of time travel would suggest otherwise. The fact that someone can be aware of a change in the timeline, implies that, at least for them, there were times when an effect predated a cause.
Actually I'm positing a 2-dimensional graph here. Treat the Y axis as time, moving upward from past to future, and the X axis as, for want of a better term, probability. Different timelines exist alongside each other as parallel vertical lines.
We also have to get into the concept of a worldline, which is a term from relativity referring to the path an object or a person follows through space
and time -- or in this case, through time and probability. Normally a person's worldline will move straight upward through the timeline they occupy. But if they go back in time, then their worldline will loop back downward, going in the opposite direction from an objective observer's worldline, so they will perceive "before" and "after" differently from other observers; what is "after" for them will be "before" for everyone else, because they're going backward. If they then end up in the past and "change" history, i.e. alter probability, then they move sideways along the X axis into another timeline, and then move upward again through it -- but in parallel to the timeline they originally came from.
This is where it's crucial to remember that perceptions of space and time are relative to the observer. You're using terms like "when an effect predated a cause" as if they were universal absolutes, but there are no such things in physics. Everything is relative to the observer. "Before" and "after" are just different points on an individual's worldline. A time traveler's worldline includes backward loops, so that will make their perceptions of "before" and "after" differ from everyone else's perceptions. If you go through life normally, 1955 will come before 1985 will come before 2015. But if you travel through time and loop your worldline back on itself, then you can perceive 1955 coming after 1985 and 1885 coming after 2015. (Just to toss out some dates totally at random...

) But that doesn't mean they "actually" take place in that order, because there is no absolute right or wrong frame of reference. It just means that the time traveler perceives them in that order while someone moving linearly through time without the benefit of a flux capacitor will perceive them in a different order. And the student of temporal physics examining the whole pattern of travels and interactions will have to abandon any absolute sense of the passage of time and see all times as just different points on a graph, and "before" and "after" as just successive points along a worldline.
Now, I admit that is based on a subjective observer inside the timeline. But if you take a step back, and look at space-time as a whole, you should see a second time dimension in which all events happen. Someone outside space-time should be able to see a series of events leading up to TNG in which the Borg never traveled back to earth, and "then" a new series of events after they had.
But those two things are not before and after each other. They're alongside each other, existing simultaneously. It's only from the perspective of an observer whose worldline moves from one to the other that one appears "old" and the other "new." By insisting on using those words as if they had absolute meaning, you're trapping yourself within time and thus can't understand how it looks from.
Also, it doesn't follow that there ever
was a timeline without interference just because you weren't aware of it. Because you're blinded by a temporal viewpoint, you make the unquestioned assumption that any time travel is a "change" from what came "before," but that's not true. The laws of temporal physics absolutely do allow for a self-consistent time loop where an event in the past was
always caused by an event in the future. Sure, it defies our intuition of causality, but that's the whole point -- our intuition evolved for linear time and is useless and misleading when dealing with time travel. Before you do anything else, you have to take a good look at your own concepts, figure out what it is you're taking for granted and assuming without question, and toss out every last bit of it.
Sure, there
can be situations where there's one timeline that's unaffected by time travel and another, divergent branch that is affected by time travel. But that isn't the
only possible circumstance. A timeline that was
always the result of time travel is completely permissible.
In fact, strictly speaking, it's the only kind of time loop that would be allowed to exist physically. Quantum physics says that you're already correlated with the events of your own future, so if you go back in time, you will correlate the past with that future and ensure that it's the future you bring about. So time travel that creates a new history is a fictional conceit that probably can't really happen, whereas time travel that self-consistently causes its own past (and "always" did) is the one kind that
is theoretically possible in real life. So the common fictional conceit that it can't exist, that any and every time travel
must be a change from some pure "original" history, is getting it backward.