The answer to the first question is “Yes” UK and or ANZAC may attack the Japanese on their first turn. This move will bring both ANZAC and the UK into formal “War” with Japan, but the US is uneffected. At this point, Japan may attack anything with Chinese forces, UK forces, ANZAC forces, and either an ANZAC symbol or a UK symbol on it without the US “going to war.” However, if I interpret the rules properly, Japan could not occupy an unoccupied Dutch territory without bringing in the US player.
The answer to your second question is “No.” The US may not attack Japan freely until their fourth turn. However, there is really no reason for the Japanese to wait beyond their third turn to attack the Allies since the US gets the +40IPC boost in their third Collect Income phase anyway, whether Japan has attacked or not.
In my extremely limited experience so far, Japan’s only liability in the beginning is their small number of transports. Make sure to check the National Objectives when deciding what to do each turn.
Enjoy the game.