Hitler did not even ask for it.
Actually, he did...several times.

So you are right, the Germans could have done much better, but it was not a realistic option for Hitler.
Of course, in reality, we all know utilizing human resources from the Ukraine was not an option considered by Hitler. But we are talking what if's here, and all of this discussion is just whimsical musings

The alliance between Japan and Germany had only one reason for Germany: Hitler believed, that the US would hesitate to step into the war against Germany, if they have to fear the war in the Pacific. Japan also hoped that the alliance with Germany would keep the USA out of the war.
This is also an accurate view. But in musing whether the Axis coalition could have done better, one area that could have made a large difference, IMO, is better co-operation between Japan and Germany...starting with better communications. The German pact with the SU in 1939 was both a surprise and a bitter pill for Japan to swallow. When Germany attacked in June of 1941, this was also a complete surprise to Japan and did nothing to further relations between the two powers. Japan's non-aggression pact with the SU was the flip-side of dampening relations with Germany.

Germany would have had to give Japan something substantial in return for a commitment for Japan to attack the Soviets in the Far East. My view on what that was has already been stated.

Would FDR have been able to declare war against Germany?
The Germans were always considered to be the larger threat, but America was not very prepared as of yet to go to war, and so was content, for the moment to supply money and materials to GB. Pearl Harbor changed all that.......