"Back then, the war was real and desperate, and winning it would prevent an invasion of Japan that would take years and cost hundreds of thousands of Allied lives (and many more Japanese than that)."
An invasion itself only made truly inevitable by the doctrine of unconditional surrender, from what I've read.
As for the idea that Ted Knight's guilt is anachronistic, that just seems wrong. Off the top of my head, Leo Szilard is one who both played a role in the Manhattan Project and had grave reservations about the use of the weapon:
There are other similar cases, if I remember correctly, so imagining that some other scientists were merely feeling guilt or remorse after the fact doesn't require any wrongheaded projection of "modern sensibilities."