The reason that you can't find a site that will tell you this as a matter of fact is because no one can possibly know whether the Holocaust could have been prevented. We can never know what might have happened if things in the past changed.
For example, let's look at one choice for how the Holocaust could have been prevented. Let's assume that Hitler got killed (as he well could have) in World War I. If there's no Hitler, there's no Holocaust, right? But we can't know that for sure. We know that there was already anti-Semitism going on in Germany and in other parts of Europe. Hitler didn't start that, he just tapped into it. So how can we know that some other person wouldn't have been able to come in and tap into that anti-Semitism and cause the Holocaust? It doesn't seem likely, but the whole Holocaust seems like something that should never have happened in any rational world too.
Here's another scenario. Once the Nazis' abuses of Jews (before they actually started killing them) came to light, other countries like England, France, and the US could have attacked Germany and overthrown the Nazis. Presumably that would have prevented the Holocaust. But would these countries ever have done that? None of them wanted war and none of them particularly loved Jews or cared about Jews having their civil rights taken away. So this is something that could have prevented the Holocaust, but it seems unlikely that it could have happened in that time and place.
Many, many things could have prevented the Holocaust. But would they have prevented it And if they would have, could they realistically have happened? We'll never know.
So, for this project, I think you should look at various scenarios that could have prevented the Holocaust, like the ones I've laid out here. Talk about why they might have prevented the Holocaust, but acknowledge that we can't ever know.