What is Britain had not entered WWII

[ReluctantSamurai]

The fear of the Bolsheviks was probably stronger than the fear of the nazis at this point, there could have been a war against the USSR over the Winter War or at the very least substantial material aid to Finland.

I don't believe this to be the case. Hitler, and his steadfast anti-Semitic policies, was perceived as the #1 threat by the US, the UK, and France. Stalin, of course, knew that sooner or later, the Soviet Union and Germany would come to blows...he was trying to make it as "later" as possible so his re-armament, and organizational plans could be in place first.

In 1938, Churchill wrote a memorandum to Lord Halifax (Britain's Foreign Secretary), urging closer co-operation with the Soviet Union, the US, and a possible united front of France, the UK, the Soviet Union, and, at the least, a benevolent neutrality with the US. For it's part, the Soviet Union informed France that it would go to war with France against Germany during the Sudetenland crisis, and that it would support a Czech request for League action if France did not honor her treaty obligations [quoted from FDR by Conrad Black].

After Bohemia and Moravia were annexed in 1938, the Foreign Policy Committee of the British House of Commons called for conscription, an all-party coalition government of national unity, and an alliance with the Soviet Union [FDR]. These views were also held by Britain's principal Dominions (Canada and Australia).

In the spring of 1939, the Soviet foreign minister, Maxim Litvinov, called the British ambassador to Moscow and gave him a proposal for a tripartite defensive pact between the USSR, Britain, and France. According to the proposal, each would be obligated to go to war to support any of the others in case of attack and to defend Poland, Romania, and Greece if any of those countries were attacked. [FDR]

It appears to me that even before Poland was attacked, that Germany was perceived as the primary enemy by all involved, and that alliance overtures were made by each. (Now there would be a much more plausible what-if....a USSR/British/French alliance going to war with Germany)

Material aid was considered and actually planned for the Finns, but thankfully was never carried out as it would have been crushed either by the Soviets or the Germans, and would have gone a long ways towards pushing the USSR and Germany into an alliance (at least for the short term).

A war via volunteers or a naval embargo of the USSR would have been a favorable recourse for the UK and France to appear strong against totalitarianism to try and regain political credibility after the failure to protect Poland

Would have been a disaster for the very same reasons cited above. Bolsheviks and Nazis would certainly be an odd couple, but you can't box them both in and not expect them to co-operate with each other on some level

There would be strong support in the West for a war against the USSR and I think they'd more likely have participated in such a war alongside Germany than standing aside as the Germans took European Russia for themselves. I can't imagine France or Britain going to war against Germany in support of the USSR in any scenario

Simply not true, IMO, for all the reasons I've cited. I'd be curious as to what leads you to believe that either the US, the UK, or France would even remotely entertain such an idea

The issue of Japan is certainly a tricky one, I can't imagine they'd fight the British Empire, France, and the Dutch if those countries weren't already at war with Germany and Italy. The forces that those nations could bring to bear against Japan if not busy with Germany and Italy would be quite tremendous

Only Britain was in any kind of position to bring forces to bear against Japan. The Dutch had only minimal forces covering the DEI, and France had even less. Britain had two major naval bases, Colombo on Ceylon and Singapore in Indonesia. Both have extremely long and vulnerable supply lines, and neither was in any sort of readiness to resist the Japanese (the Malayan campaign took only six weeks concluding with the surrender of Singapore on 15 Feb 1942). With only 150 or so front-line aircraft (the Brewster Buffalo as the main fighter) ) and Force Z consisting of two antiquated BB's and four DD's at the ready, the outcome is a foregone conclusion (as events showed). Colombo is probably outside Japan's logistical reach, but her subs are much more capable of interdiction than Britain's, and could conceivably render the base useless.

I think it's more likely that Japan would have essentially sued for a favorable peace in China than to fight the Imperial nations in a bid to try and secure oil

Given that Japan needed access to both China's and Korea's coal for making coke (steel-making folks...not the other kind), why would they agree to this? To say nothing of losing face to what Japan considered as a sub-human race...

Japan would as you said probably have turned North instead and participated in a German attack on the USSR

After the debacle at Khalkin Gol in 1939, only the extreme IJA hardliners still favored war with the USSR. It had become painfully obvious that Japan's deficiencies in armor, artillery, and especially in unit mobility would make any venture against the USSR costly until those deficiencies could be rectified. (In fact, Japan's AGS felt that an initial breakthrough could be managed against the Soviets, but exploiting such a breakthrough would be extremely difficult due to Japan's lack of trucks and mechanization.) Personally, I don't believe they could even manage a breakthrough in any kind of terrain where Soviet tanks could operate

What would Japan stand to gain other then a very long casualty list? The oil, bauxite, and rubber in the DEI was much easier to acquire against a much weaker opponent. Siberia/Mongolia has lots of.......well, one has a lot of very dry badlands, the other a lot of snow and ice

One other note; the German attack on the USSR came as a complete surprise to the Japanese as well as the Soviets. Japan's closest ally hasn't even bothered to inform her of Germany's plans to invade