Actually, now that I think about it, the American political situation might be radically different in such a scenario. Perhaps not in a way that would ultimately prevent war with Japan, but certainly different enough that it should be factored into the scenario.
FDR dithered until the very last possible moment as to whether he would run for a third term or not - breaking precedent, after all, was a big deal. His main justification was that America needed strong, stable leadership at a time when it seemed increasingly likely that the country would get involved in a massive global conflict. Now, the geopolitical situation will still be tense by the time that the Democratic National Convention rolls around, but if the war in Europe seems at least paused, then FDR might not feel as though he could get away with running for a third term. The nomination seems most likely to go to John Nance Garner in such a scenario, though there are any number of other possibilities.
On the Republican side, Wendell Wilkie’s triumph at the Republican National Convention was an absolute black swan event that seems very likely to be butterflied away under even slightly different circumstances. The most likely alternative nominees are Thomas Dewey and Robert Taft, both of whom were at least nominally isolationists at the time. Dewey could very well change his tune once in office, but Taft was absolutely, ideologically committed to avoiding what he perceived as foreign entanglements. That would certainly complicate matters.
But how likely is a Republican victory in the general election? I think that it is fairly likely in a scenario where FDR is not running as the incumbent and war is not currently raging in Europe and the Atlantic, especially if Garner is the nominee. Left-wing and/or black voters would likely be less enthusiastic for a conservative Southerner than they were for Roosevelt, and this could provide a decisive margin for Dewey or Taft in the Midwest and Northeast.