Science is the human activity where previously unanswered questions are probed through systemic methods based on logic and rationality. Experiments and observations allow the creation of theories. Those theories offer predictions, such as the constancy of gravity. So no, we don't know gravity is constant because it is so each time we measure it. We know it's constant because the best theories that explain the mechanics of its workings predict that constancy. If observations were to contradict the predictions, then we must evaluate the details of the explanatory theory.
Spend a couple of minutes evaluating what you said, and you'll find the mistake in your logic. We did not actually see the big bang happen. It happened BILLIONS of years before we even existed. Yet, we are able to extrapolate the likelihood of that event based on the current state of the universe. Similarly, one can hypothesize extra-cosmic conditions that can give rise to big bangs. Mathematics are incredibly powerful and sophisticated, and they are perfectly capable of examining such abstract concepts.
So the conditions that existed before the big bang are just as much the realm of rational inquiry as the conditions that lead to a tornado. In fact, ALL achievable answers lie within rational inquiry, since no other method has shown to do anything. After all, we had your religion, and others like it, for tens of thousands of years. And in a few hundred years of science, we managed to surpass the achievements of our forebears a million times over.
And why should it be any other way? Why should there be a cut off point where serious study is no longer sufficient, leaving unjustified random feelings as our tool of choice to solve unanswered riddles?
In short, can rational inquiry find a solution to every problem? Maybe, or maybe not. But if science can't find an answer, then nothing can. That is a simple fact, since the alternatives lack a convincing meaning outside of their faith-baseness.
This post is intended to add to the discussion in general, so don't feel obligated to respond point by point.
I don't think you can assume that to be true. Nobody knows what the conditions are like before all matter, space and time ever existed.