Per e.g. the facts that Rhodes lays out, if the Germans had not screwed up organizationally, had not incorrectly dismissed graphite as a moderator for natural uranium reactors, they might have succeeded with the plutonium path. The fear was legitimate, and per Groves in Now It Can Be Told (he was in charge of counterintelligence), it took us a long time and quite some distance into Western Europe to be fairly confident they weren't doing it. So, no, not the last week, not even 1945, but indeed for a long time.
This. What mattered from the standpoint of US decision making wasn't what the Germans were actually doing, but rather what the US leadership thought the Germans were doing.
Or more specifically, from an intelligence viewpoint, what the Germans could be doing. Intent was well established when Hitler declared war on us (big mistake), but before that always comes capabilities. They had the uranium (something that Japan never had), they had the industrial capability for the plutonium path (ditto), and they most certainly had the scientists (even after chasing all their Jews away). If they'd gotten serious about it sufficiently earlier than we did....
They might have nailed down the science, but not the efforts necessary to gather the fissile material. And then, even if they did have the resources, actually doing all that so close to the action and with the risk of bombing by air or spies, would be inconceivable. The american, actually the canadian-american project as that's where the uranium came from, had the advantage of vast open spaces well outside the range of any enemy aircraft.
the efforts necessary to gather the fissile material
That's what the plutonium path is all about. It doesn't require a lot, and between the uranium ore from Africa they got when the captured Belgium, and the not as rich ore from Czechoslovakia, it was judged it might be enough.
You say it's "inconceivable", but it was indeed "conceived", considered to be possible, if perhaps unlikely, by lots of scientists and engineers working on the Manhattan Project. Who as it progressed had a better and better idea of exactly what was required. And who knew how long we dicked around with the concept before committing to it, and then putting Groves in charge.
There's also the danger of 20/20 hindsight. For example, the critical March 1940 Frisch–Peierls memorandum (https://en.wikipedia.org/wiki/Frisch%E2%80%93Peierls_memoran...) laid out the scientific path to success (fast fission) and estimated about 1 kg (2.2 lb) of U-235 was required. Later it was learned it was 52 kg, and plutonium-239 a much more modest 10 kg. Suppose the universe instead allowed 1 pound of Pu-239? Is that still "inconceivable"?
Was the V-2 effort "inconceivable"? It required a similar order of magnitude of effort, we knew more about the project because they had to test fly it, the threat was obvious and it did cause a lot of harm, and yet the "risk by air" was not enough to take it out prior to that harm.
The final detail is that even if low priority (you're not going to convince many it was truly impossible), the consequences of their getting it before it was too late for the Third Reich were ... well, it would have been existential for them.
The final detail is that even if low probability (you're not going to convince many it was truly impossible), the consequences of their getting it before it was too late for the Third Reich were ... well, it would have been existential for them.
