Just few thoughts concerning the Red victory and Finland becoming a part of Soviet Union:
One major problem with the Red victory in the Civil War is the fact that the Reds were significantly less organized than the Whites. Unlike the Whites, they also lacked experienced and trained soldiers and officers.
It’s also worth-noting that many Reds weren’t prepared for the full-scale war. They understood that there will be fighting, but it seems that they overestimated their support and Finnish population would just flock to them after they have made their declaration. You probably wouldn’t have had early rebellions in places like Oulu, Kemi and Tornio in Northern Finland, if the Reds had realized how things would go.
By early 1918 the White Guards have been also able to gather more weapons and other equipment than the Reds. The situation in this regard had been more equal earlier, but especially after the declaration of independence the gap had been increasing for the benefit of Whites.
Above mentioned reasons meant that the Reds, apart from some very early attempts, were unable to mount any significant offensives during the war.
DrakonFin has already mentioned things which might solve some of these issues. If some key personnel in the White leadership had died/been captured, it might have helped the Reds. It’s important to note though that the Whites did have a quite large number of capable leaders so they can probably lose some and still have enough good leaders to win the war. Somehow getting more jägers who had served in the German Army and former Imperial Army officers to defect to the Red side would help them very significantly, even if numbers were limited.
The Reds need to move quicker than they did in our history’s civil war. Taking Mikkeli and Sortavala in Eastern Finland might have been doable even IOTL Civil War, but the Whites moved just little too quickly and the Reds little too slowly for that to happen. Better preparations and planning might solve these issues, especially if the Reds have slightly better officers and the Whites worse as I proposed above. If Germany doesn’t do its intervention and Lenin gives more help to the Reds, that might help.
I must add though that even with all this happening, I would probably still put my money on the Whites winning. The Reds could probably win a civil war with an earlier POD, but the civil war seems somewhat unlikely, though not impossible.
I pretty much agree with this. Any realistic changes we can make into the OTL civil war after it is underway would most likely not be enough to avert a White victory. The military skill and ability on the White side was so much better that it was a decisive factor.
However, several bumps in the road could be created by seemingly minor changes and flukes. Consider, for example, that during the first night of the Red takeover of Helsinki, the revolutionaries manage to catch the entire Svinhufvud senate through a massive stroke of luck. Let's say Mannerheim suffers a fatal accident during roughly the same time - an attempt to disarm Russians in Ostrobothnia goes awry, and he gets shot.
This alone would mean that the Whites would be decapitated and without such a strong claim to legitimacy at a crucial time. As a result, building the White military and the bureacratic apparatus of the White political leadership would take longer and be wrought with disagreement over who is in overall control. In the meanwhile, the Reds would have more time to shore up their hold on to southern Finland. Let's say that the Reds go on a general offensive a few weeks into the war, while the White side is still in a comparative disarray, and that due to (handwave), the Bolsheviks are able to send them real help in the form of actual battle-hardened soldiers and revolutionary officers. In such a scenario, essentially a White-screw and comparative Red-wank, a skilled writer could probably cobble together a
seemingly realistic Red victory scenario, provided that all gods of war are continually on the Red side and that the Whites meet setback after setback.
More realistically, though, I am also of the opinion that pre-war PODs would be needed for a non-contrived, truly plausible Red victory in a Finnish civil war.
Regarding the effects of the Red victory, you would probably see the Finnish Socialist Workers' Republic being one of the states signing the treaty to establish the USSR besides Russia, Ukraine, Byelorussia and Transcaucasia. Most Reds weren’t aiming towards the dictatorship like there developed in Soviet Russia but the international situation would eventually force them to get closer to Moscow, especially if the Soviets had helped them during the war.
The Soviet control of Finnish southern coast (especially places where they did have bases IOTL, like Hanko and Porkkala) and Gogland and other Islands in the Gulf of Finland would significantly increase the geopolitical position of the Soviet Union and help to ensure the safety of Leningrad. Having Åland would be also very important, but it’s quite possible that Swedes take it in a scenario like this.
One interesting effect of the Red victory in Finland are its spillover effects elsewhere. The Estonian Independence War was a close-run thing and the lack of Finnish support might be just enough to turn the tides. This will also help the Soviets in Latvia too, though the effect probably lessens further south you go. Lithuania and Poland might go actually pretty much as IOTL, though I might be wrong.
Soviet Finland would probably get a Russian minority significantly larger than we have now. On the other hand, Swedish-speaking minority will probably suffer, especially if we get a leader like Stalin even ITTL. In the best-case scenario, you might get something like what happened to Karelians and Finns in Karelo-Finnish SSR during the 1940’s though; Stalin somewhat lessened oppression because he realized that it might have a negative influence on the Soviet reputation in Finland*. That depends how Soviet-Swedish relations develop of course.
*There were originally plans to move all Karelians and Finns in Karelia to Central Asia following the WW2 similarly how for example Crimean Tatars and Chechens had been moved moved but Stalin dropped them off from his eventual plans.
Like I have commented before, a Red victory in Finland could have significant knock-on effects in the Baltics (we could posit, say, a domino theory beginning in Estonia, where instead of White Finnish help there would be some
Red Finnish help), maybe Poland and then in the Soviet state itself. We might see a *USSR that has comparatively effortlessly taken Finland and the Baltics, and is going strong in Poland, deciding that exporting the Revolution is something that will work, based on these early successes. A Soviet Union that stays internationalist and avoids "Socialism in one country" might make the 20s and 30s really different from the OTL. Also the different wars and events of 1918-21 would mean that different Soviet leaders would live or die. We could plausibly bump off Trotsky or Stalin this early, say, and keep alive men who died in this time IOTL, perhaps making some of them future leaders of the Soviet state, reformers even.
The Red Finnish/*Soviet control of Finland would mean changes in Sweden's politics, and seeing something like the OTL Åland Crisis between Sweden and the Finnish Workers' Republic or the *USSR might be quite interesting. Sweden might well get the islands, but then Red revanchism towards Sweden might well be inevitable, too.
The point about a Russian minority in Finland is a good one (also like
@Karelian said above), and like always, we need to remember that these Russians coming to Finland would not only change Finland - they would also change Russia, even if in smaller ways. Maybe the *Finnish SSR will get Bolshevik administrators that survive due to this posting, using it as a stepstone towards (comparative) greatness, or maybe people who became prominent IOTL go to Finland to get buried in bureaucratic mediocrity. On the other hand, Finnish Communists might have better careers in the *USSR due to the Red victory in Finland, some of them being seen as early heroes of Communism, and especially if this *USSR does not go Stalinist or involve similar purges of minorities, we might see more and even more successful careers for Finns in the vein of O.W. Kuusinen.