IF Japan was not in discussions to join Axis would Germany pressure Vichy regime to allow them into Indochina? or just take no position? not sure what options French had? cooperate/collaborate with KMT China? (this was same month they fought off British force at Dakar but they did not have that type of firepower in Indochina)
IF Japan did not join Axis, Germany might have continued their unhappy collaboration with USSR?
in Sept. of 1940 Germany had just sent first raider Komet thru Northern Sea Route to Pacific with consent and help of Soviets and were discussing using ships using other direction to carry freight as well as repair base in remote Kamchatka. the Soviet-Japanese Neutrality Pact had not been signed at that time (not until April 1941.)
The French were, from the Japanese PoV collaborating with the KMT. The port of Haiphong was becoming a alternate to the Chinese ports the Japanese held.
Conquest of the easter slavic populations was high on Hitlers to do list. He had expected to do that very soon anyway, before any war in the west vs either France of Britain. The collaboration between the USSR and nazi Germany was as you say unhappy, and it was understood the conquest would be more difficult later.
in the autumn of 1940 the decision was made to attack the USSR in the spring of 1941. I'm unclear what the point is about the Soviet-Japanese Nuetrality Pact is.
the Vichy regime was trading with KMT, the possibility exists they could increase that, making the Japanese invasion counterproductive ... not that
that was something stopping the Japanese in all instances!
yes, understand the clash with USSR was considered to be fate but there were serious discussions for Soviets to join Axis. meant that there was period where Japan would not have concluded any treaty or agreement with either Axis or Soviets (they are "free agent")
Germany would desire a Soviet-Japanese war, they were not able to entice Japan into that historically, even after giving up their dealings with KMT China and pressuring the Vichy in Indochina. while unlikely, they might have had more success making Indochina appear
bitter fruit and serving as Soviet
cat's paw in Pacific? leading to the Strike North.
(even if unsuccessful they have maintained their trade with USSR, China, Indochina, Iran, and solidified their collaboration with Vichy France)