But why wouldn’t the US then also get the 30 IPC bonus income for holding western US at the start of their turn? (getting 60 bonus IPC).
The 30 IPCs for controlling Western United States, unlike the one-time bonus for an unprovoked declaration of war by Japan, is from a National Objective. It, like all National Objective income, is collected in the Collect Income phase (at the end) of the power’s turn.
The rules for declaring war state that it’s technically not required if they declared war on you “as the two powers are already at war” (see below extract from page 11). Though I feel like this must excludes the US and they must “declare war” during their combat move despite already being at war?
No, it’s not necessary.