Would they accept separate allies surrendering? Would the Japanese/Germans accept a surrender to one Axis but not the other? What were the occupation plans of the United States?
Germany, as such had no official plans for an invasion of the United States.
Originally, when war was declared by the United Kingdom, the German High Command believed that a mainland invasion of the British isles was plausible and drafted Unternehmen Seelöwe (Operatino Sealion) which was a full amphibious and airborne invasion of the UK. However in order to stage this, the Luftwaffe needed to have full control of the skies in Britain in order for the airborne forces to have the go ahead. Hitler gave a directive for this in that the RAF was to be "beaten down in its morale and in fact, that it can no longer display any appreciable aggressive force in opposition to the German crossing". The Luftwaffe then began, in the summer of 1940, heavy daylight and night-time bombing of British air bases and industry in Operation Adlertag (Eagle Day). However, Hitler's forces were repulsed conclusively by the Royal Air Force's fighter command in what is known as the Battle of Britain. If you want any further reading on this, I would recommend 'The Battle of Britain' by Charles Messenger, Bison Books.
When the US joined the Allies in 1941, the Oberkommando der Wehrmacht (German High Command), recognised that the current strength of the Wehrmacht would make it almost impossible to invade the US directly. The focus was put instead on the Kreigsmarine to direct attacks by U-boat in order to disrupt American Merchant shipping, specifically to the UK in Operation Drumbeat.
Adolf Hitler initially dismissed the US as a realistic threat, stating the country was "made up of Jews and Niggers" and thus had little fighting strength. Other members of the Oberkommando did however realise the huge industrial and productive power of the US and, in 1942, briefly investigated a Transatlantic attack but abandoned the plans because there were too few possible staging bases for such an attack in the Western Hemisphere. Germany decided she would wait to see the result of the American-Japanese conflict, before going any further with plans for an invasion. Later in 1944, the Germans acknowledged the fact that any attack would be suicidal and instead starting gearing military strategy towards surrendering to the US, to escape destruction from the advancing Red Army.
Japan, lacked both the manpower or the support to launch an attack on the mainlands, but did briefly occupy the territory of Aleutian Islands of Alaska.
I'm really sorry, I wrote a long answer to this, but my iPad died and it was lost just before I was gonna post it. I'll post another answer tomorrow.