It sure dont work that way in the Pacific, if Japan attack neutral DEI, then Japan is suddenly at war with everybody, even neutral USA.
Not immediately. Japan is only at war with UK/ANZAC. The US is allowed to declare war on Japan on its next turn. The same thing applies here. If Italy declares war on the USSR, the USSR is allowed to declare war on Germany on its next turn, but it is not immediately at war with Germany.
And if it was the other way around, that Russia attacked Italy, then I am pretty sure that Russia would automatically be at war with Germany also.
No, but Germany could declare war on the USSR on its next turn.
If Italy attack USA, is only Italy at war with USA then ? Germany is not ?
Correct. However, either Germany or USA may declare war on the other on their next turn.