That's quite literally how it's performed. You make a hypothesis, then you test it. If it fails, you alter the hypothesis and try again. A single rocket exploding once every couple months is absolutely nothing to the planet compared to all the other activities humans do.

They aren't just randomly lobbing these into the sky over populated areas (like China likes to do). The paths they follow are very much planned to account for accidents. That's why they launch them over oceans. And they will deliberate blow them up using what's called a "flight termination system" if it looks like things will get hairy.

It has nothing to do with science, it's Engineering.

The upper stage exploding is of course not ideal, but it does so on a suborbital trajectory, meaning everything will fall back down to Earth, into the ocean. (these still being test-launches with certain risks is the whole reason they're choosing not to go fully orbital yet)

In terms of materials, it's mostly just stainless steel, there is no pollution from it.