The “Atlantic Charter” proclamation refers specifically to looking toward a world after the destruction of “Nazi tyranny” — it’s great that FDR was bringing to US to such a position in an official diplomatic document, but it was certainly outside the norms of neutrality.
It is a fascinating (and harrowing) period between Sept. 1, 1939 and Dec. 7, 1941 as FDR and other US leaders increasingly recognize the need to align the USA in support of the UK (and then even of the odious USSR) in order to defeat the Axis powers, even while the USA is not officially in the war.
Without the Pearl Harbor and related attacks by Japan, who knows how long the USA might have continued in the murky terrain between peace and war, trying to provide (”lend-lease” etc.) enough supplies and weapons to allow the UK and then USSR to resist, while not declaring war upon Hitler and Naziism.
Indeed, if Hitler had not been so foolish as to declare war on the USA on Dec. 11, 1941, would FDR and Congress have opted to go to war with Nazi Germany, at least right away? Or would we have been solely engaged in fighting a Pacific Ocean war against Japan while continuing to supply the UK and USSR?
To your last point, I would say that it would have been very difficult for FDR to get Congress to declare war on Germany as well. I think it still would have been an eventuality, but it may have taken months or perhaps even more to convince them to fight on both fronts. I’m pretty certain that if any significant amount of time passed between the declaration of war on Japan and the request to declare it on Germany, there would have been a large contingent protesting the prospect of fighting a two front war and a “Not until Japan is defeated” crowd would have taken hold not only among the populous, but in Congress as well.
I think we would have ultimately provoked Nazi Germany into war by continuing to ship to Britain, and the first time a US ship was fired on, would have provided us with the Casus Belli to declare war on Germany.