But their nuclear physicists were good enough to distinguish between the residue of a uranium bomb (slow but relatively easy to make) and plutonium bomb (harder to make but potentially very much quicker to churn out).The soviet invasion and the two atomic bombs all were within three days of each other, you can't split them apart. the reality is it's not either/or, bomb or soviets, but both
The IJA didn't really care that much about the bombing at home certainly not the commands abroad.
The home authorities didn't really care too much about the red army in Manchuria
That's why it took Hirohito surrender addressing the nation and talking about the bombs, and then when the IJA didn't surrender after that it took a specific command (plea) to the IJA abroad to surrender that mentioned the Soviets
(and frankly even than there was an attempted coup because while the IJA talks about loyalty to the emperor it's only really when he says what they want to hear, and when ever they don't hear it why it must be because he's being misadvised by traitors and must be saved!)
In regards the threatened invasion of Japan that was actually the Japanese plan! The Japanese high command felt that if the US and UK & Co invaded they could inflict so many casualties on them that they would be forced to compromise on the unconditional surrender demands and come to the table or retreat whish would more politically unpalatable for the wallies, the whole thing possibly being facilitated by a neutral USSR
But both the atomic bombing and the USSR declaring war and them steamrolling over the Manchukuo army caused issue for that plan
1). The atomic bombing* what they knew was unlike mass bombing raids that required large numbers of planes and aircrews and specific situations to cause mass damage and death, now seemingly a single bomber could do the same. This presented them with the potential reality that unlike years of conventional bombing in Europe and months in Japan it was possible that country could now be functionally destroyed from the air, no invasion needed
2a). teh USSR was clearly not Neutral anymore so the USSR acting as impartial uncommitted facilitators was done
2b). While Japanese world views might still have allowed them to think the US and UK & Co were shy when it came to casualties, they had no such illusions about the USSR being willing to to suffer them if they became involved in any invasion. They also know that once the red army is standing on something the USSR is not going to let go of it
EDIT: there is also the point about the ongoing blockade, there was already food shortages on the main island, teh IJA probably wouldn't have cared that much. They know they will first in the food line, and had the attitude that the civilian's role was to primarily to give teh army what they needed to resist and then endure in order to eke out some kid of positive result. But once starvation starts kicking in it will kick in hard and fast
*and we have to remember the Japanese did not know how many bombs the US had, what production lines was like and so on. What they did know as was the thing their own guys had said was not feasible as a deliverable weapon was now deliverable as a weapon,
How much influence they had on the decision makers, I can't say, but at least some senior military and political figures would have know they faced production line destruction.