There were actually other issues. Germans were given military control over the northern part of the country and had number of troops located there. And therefore the Germans held small part of population (and country) as a sort of a hostage. Furthermore Finland was not self-sufficient at the time and was dependent on the foreign trade on food just to survive. The USSR could not be trusted (due to the Winter War and subsequent events) to do this - after all the USSR had already in 1940 tried to blackmail Finland into political concessions using food shipments as an extortion tool. So the main route to Finland was across the Baltic Sea (in fact in many ways Finland was like an island in this respect). With Germans controlling Denmark and Norway this pretty much meant that the Germans would have been in control of it.
Had the Western Allies intervened in Norway the situation would have been very different. More on this: Finnish plans if allies land in Norway? - #2 by WandererRTF
Also the problem with allowing the Soviets ‘to help’ was that they had tendency of not leaving afterwards so despite of the threat posed by the Germans i doubt the Soviet ‘direct support’ would have been accepted.