How could the human race have gone back in time to save itself from circumstances that would have caused us to go extinct without our own help (the convenient wormhole)?
The reason this is so difficult to answer is that you've included the words "back in time".
There is no linear time in this movie. There is no past, and no future. There are creatures, primarily humans, in the movie which experience time linearly, and have no ability to move around in time except at a fixed (perceived) pace.
Consider two walls on a road, traveling like vehicles, spaced apart from each other about 3 feet (1 meter), moving down the road. You are between the walls, and must move with them down the road. You do not have the ability to go around or over the walls, nor visibility to see what's beyond them - only what you see in the 3 feet of ground, and what's above and to the sides. You suspect that the walls aren't infinite, but you don't have the tools or ability to climb over or around the walls. So you are pushed forward, and you can't go "back" to places you've already been, nor "forward" to places you'll eventually come to.
Now change it slightly. Put the walls in the ocean, and put a fish there. It doesn't even perceive the need for tools or techniques to go over the wall - it just swims up and over. In fact the wall isn't an obstacle at all. It can fully utilize all three dimensions, and even when resting there's no need for it to move at all along with the walls.
The walls are irrelevant to the fish.
We perceive time as these walls. We can't travel to the past or the future. We don't have the ability to stop the walls or move outside the walls.
But a being that perceives time in the same way we perceive places can go to anywhen, the same way we might travel to anywhere.
In the same way that you have the monitor placed on your desk in a certain position so you can use it - it belongs there - the wormhole is placed in a certain when. That when is neither "before" or "after" any other when. It's just a when, exactly the same as your monitor is not "before" or "after" any other where. It's just a where. All where's exist without an ordering. You might perceive two monitors, one in "front" of the other as having an order, but if you move yourself around them, the order changes and isn't any less or more relevant.
So all whens exist. Some beings perceive whens in a particular order, but that doesn't mean that these whens actually have an ordering anymore than we see wheres as having an ordering.
Once you move past that perception and accept and perceive that all whens are unordered, the same as all wheres are unordered, then the wormhole exists in that when and where. Someone created it, and put it at that when. There was no before or after it was placed - it's placed at a when, and exists there. It will "always" exist there, despite the fact that "always" has lost meaning since it has never not been at that where and when.
In other words, 4 dimensional humans already exist "now", they've "always" existed, and will "always" exist. In the same way you move a glass of water from the kitchen to your desk, they created a wormhole and placed it at a when and where. It's there at a when. We can't perceive it, "yet", but it's "already" there at a when, "always" was there at a when, and "always" will be there at a when. To them, there is no time. Just wheres and whens.
So the real question is - How did we break through or go outside those walls? Not when - it doesn't matter "when" the first human perceived time as a when rather than linearly. Once you step out of time, it doesn't matter when you did it - you have access to all the whens, and linear time has no meaning. It could be in the past for us. It could be in the future for us. It could be yesterday, today, or tomorrow - only one person has to transcend time for all of us to transcend time. Once any human moves beyond that perception, all the whens are as accessible to that human as the wheres are accessible to us.
But, of course, the how doesn't then matter. The wall was transcended. The wall is transcended. The wall will be transcended. There is no wall.
Interstellar essentially says that we are fish, though we don't perceive ourselves that way.