The remaining 5 FF would not be allowed to intercept, since the raider group is larger. No to both. Japan would have to make two declarations of war, at a cost of 35 BRPs each, if it wanted to attack both Britain and Russia. Yes to all of the above questions. Australian and Indian units may only be broken down or combined if they are first moved or SRed to their respective boxes.
Chinese resistance is zero at the start of every scenario.
Japan must sea transport, invade, airdrop or SR a ground unit into Saigon in order to take control of French Indochina. Japan may therefore not use airbases in southern French Indochina in the turn it attacks the Western Allies unless it has occupied French Indochina in a previous turn. The Sequence of Play says during the unit construction phase, and the Unit Conversion Table says at the start of the SR phase. Neither would apply to a Siberian transfer. Nothing actually happens to units when they pass from one mapboard to the other, especially during a Siberian transfer.