And if they originally had 48, so what? The fact that humans have a fused chromosome says absolutely nothing about their common ancestry with apes.

That's a bit like saying that the similarities in characters between Godfather I and Godfather II says absolutely nothing about their common ancestry.

It doesn't matter when the fusion took place, because the fusion event says nothing about whether they had a common ancestor.

For evolution as a mechanistic explanation to be credible, it has to be able to mechanistically explain whatever genetic differences are found between chimps and apes. The fact that it continually and consistently can is strong support.

If a genetic difference is found that cannot be explained by evolutionary mechanisms, that would be evidence against evolution. That's why evolution is falsifiable. For example, suppose humans were found to have 12 genes essential to "humanness" that used a completely different genetic code than that found anywhere else. That would falsify evolution.

Now consider your designer. Give me a genetic difference between chimps and humans that your designer could not produce. Can't do it? That's why your hypothesis is not falsifiable. That's why it isn't science.