Building a few more bunkers is easier than moving a city, especially in the 30s.

But to help your argument, there were plenty of propaganda pieces and Finnish communist organizations supported by the USSR before the war. So probably there were intentions of biting off more than expressed.

And to help mine (sort of, it's an appellation to authority), I think I've read many notable figures, even Mannerheim himself, considered the proposed deal reasonable.

Comparing this to Czechoslovakia, USSR still took exactly what it initially demanded, and I don't remember Nazis offering anyone anything in exchange. And comparing this to Baltics - there it was a different scheme, where IIRC their governments (small cozy authoritarian ones, which is very funny) asked USSR for protection (because Nazis were scarier), Soviet troops entered those countries and suddenly there were Soviet state institutions in place and plebiscites.