11. The impossibility of keeping this force together for a longer period of time.
This point is an argument for the supposed attack's date (in 1941, summer or autumn) as well.
The force the Soviet gathered on the west was huge. Most of manpower came from the agricultural sector, those who were employed in industry enjoyed protected status from conscription (instituted 1 September 1939). This means they deprived the agriculture of working hands while created more mouth to feed. And all this before the harvest of 1941. Feeding this amount of people, the Soviet Union cannot do. The agriculture itself was still in ruins. The production barely reached the level of 1916. So how they wanted to feed all these soldiers? They could take food somewhere else, from the rest of Europe.
Building up a fuckhueg army and keeping it together just to let it sit on it's ass is always a mistake. If they do nothing the morale will plummet. They get a bunch of idle hands, and idle hands are the devil's workshop. And these hands have weapons in them. Not a good idea. A force like this meant to be directed to a target and used.
When the FSE was set up and grouped to the west there weren't enough facilities to accommodate them. Many units were ordered to quickly erect some barracks. Then they were moved out of their temporary quarters to the frontier, where everyone got into tents. IN parallel with this the units of the SSE started to arrive but they weren't billeted in the barracks FSE left behind also were placed into tents.
Planning to leave this force idle during winter without food would have been a very bad idea. The only logical conclusion is that they were meant to be used during the summer or autumn of 1941.

12. They know the Germans will attack but they thought later.
It was an open secret the Germans wanted to attack at one point, it was part of the Nazi rhetoric. Lebensraum, evil communists, Jews and all that. But everyone knew the Germans can't win in a war of two fronts. One was opened in the west they couldn't leave it like that, it would mean the end of the Reich. Stalin knew that. Hitler did too. Also intelligence reports came the Germans wanted to finish Britain first. So the Stalin and co. believed they had to and need to end the war in the west before they turn on the SU.
One interesting thing, the GRU's observation on the Wehrmacht. The GRU constantly monitored the price of mutton and gathered every dirty cloth the German soldiers left behind. Why? To attacking the SU the German army would needed two things - calculated the Soviet military intelligence -: 1. sheepskin overcoats for the soldier's winter equipment; 2. oil that doesn't coagulate in the cold. Making millions of coats would need to butcher millions of sheep causing mutton prices to plummet, gathering oily rags is kinda self explanatory. So the GRU monitored this and while more and more German units were grouped to the border they reported the Germans weren't preparing for an attack because they weren't producing the material they would need to fight a war against the SU.