At the same time, Stalin doesn't realize how bad a shape his purges have left the Red Army in.
Actually Nomonhan (although victorious) did a lot to reveal Red Army's shortcomings, as well as "Liberation March" (annexation of territories East of Curzon Line) and experience of Spanish Civil War did. And Soviet leadership was very busy analyzing those lessons. People tend to think that ANY changes in Red Army, started later than 1st day of the Winter War, were direct result of the war. Completely ignoring the simple fact that huge highly bureaucratic structure of the army and Party-controlled state need some time to develop a plan for reforms, at very least. I would argue that precious few of changes started before Summer 1940 had been influenced by the Winter War.
And did Stalin really do much to improve the Soviet defences after the debacle of the Winter War?
Let's put it this way. He did a lot to change Red Army between summer 1939 and summer 1941. Were those changes for good or for bad is a matter of discussion.
Expect, then, a Soviet invasion of Finland in 1940-41. Something resembling the events that took place in a few other small nations bordering the USSR at that time.
Romanian experience proves that ceding part of territory to Stalin does not generate future demands. I understand that "we had no alternative" is a cornerstone of the Finnish lore, but it is not necessarily true.
The Soviet attack on Finland was such a naked act of aggression only the Japanese and Nazis did better.
Let me expand your knowledge:
1938 - Poland participates in Nazi dismemberment of Czechoslovakia, nibbling on it's territory.
1938 - Hungary participates in Nazi dismemberment of Czechoslovakia, nibbling on it's territory.
1939 - Hungary attacks Slovakia to get a piece of it's territory.
1939 - Slovakia teams up with Nazi to get part of Polish territory.
1940 - Hungary takes some territory from Romania, under German guns.
1940 - Bulgaria takes Southern Dobrudja from Romania.
Is Stalin too obdurate to accept an actual Ally?
Stalin was willing to accept any ally at this point. Remember, an idea of American and British forces fighting on the Eastern Front had been discussed seriously. So, would Finns offer such a deal, talks would happen. Now, I don't believe in him accepting pre-1939 Status Quo without some face-saving measure (like keeping a 100-meter wide strip of land along the Finnish side of the old border), but Finns would be under a serious pressure to reach an agreement too (they would perfectly understand that Joe's goodwill would last only while he's desperate).
Stalin invention of a 'Finnish government' in exile certainly did not suggest good motives on his part...
Yes, although it must be said that Terijoki Government was a hasty wartime invention. Very likely, it popped up only after talks collapsed.