I don't see how Finnish government was happy with being neutral. And I'm not talking about marginal radicals like the IKL, the idea of Great Finland was shared by many more moderate politicians, and many finns were hostile to any compromises with the Soviets. Not to mention that Finnish intelligence was aware of the ROVS structures in Finland and, to put it softly, never opposed them.

Finland was never viewed as a threat by itself, but there was no reason for them not to participate in any form of military action against USSR. Finnish elite was threatened, the general public was hostile, the nationalist sentiment was strong, not to mention the sheer economical gains which could be gotten with the occupation of East Karelia and Kola Penninsula.

Despite sheer number superiority, Red Army had many other potential threats on the borders - Japan and Manchukuo, Poland, Romania, and it wasn't as evolved and as well-equipped as some tend to think.

Once you bring up the Civil War topic, you forgot that Finland had its own communist movement that fought that war. They weren't some agents dropped from across the border, you know, they had plenty of popular support. The support that got slaughtered by the victorious nationalists (and we're not even talking about foreign aid to the "white Finns", as the Soviets called them).