Yes, it is possible to take DC with Japan if you pull Operation Canadian Shield or a variant thereof (like if only Japan invades and Germany does not, or something.)
Yes, it takes a while to do.
Yes, it can be done faster than I did it against a44bigdog (I just did not want to invest in the bombers to SBR DC every round.)
Yes, Russia will most likely still fall before DC (because Russian territories are mostly worthless, Russia’s besieged by two nations simultaneously on two flanks and England’s being chewed alive and cannot support as readily where as America has a few high value territories allowing them to build at least 6 infantry a round even if everything else is gone (C. USA and E. USA))
That said, I still feel that the invasion of Hollywood is more a distraction to keep America out of the war than an effort to capture America.
After you’ve accomplished your primary objective of getting Moscow and the secondary objective of securing Berlin, then you can develop the shield to take out DC, assuming you have not been kicked off the North American continent at that point.
In my humble opinion.