I think you are misunderstanding me - I don't disagree that Russia can be defeated, but it requires limited and defined objectives. 'Kick in the door and the whole rotten structure falls down' was not an objective ... it was a hope based on Germany's experience against France.
Certainly, I'd agree that German hopes in 1941 were too optimistic. They failed on two counts. Firstly by underestimating the political will of another totalitarian government. Secondly by overestimating the capabilities of their own forces, which whilst certainly the finest in the world at this point were still incredibly reliant on luck and a logistical system held together with rubber bands and sticking plaster.
But that doesn't mean that a full blown invasion of Russia by itself is impossible. As I've noted, the German example in the First World War were their armies advanced deep into Russia. At the time of the armistice they showed little sign of stopping, and up until that point the Russians had showed little ability to stop them in the long-term.
As for Germany sending Lenin back ... Lenin would have gone back anyway. My main point is that the Russian army didn't break down because of German action ... it broke down because of internal Russian politics and Germany was only able to advance unopposed because the soldiers chose to stand aside and let them do so, based on promises made by Lenin. I am not arguing that Russia wasn't defeated, I am saying that they were not defeated as a result of some grand German strategic objective, but as a consequence of actions which were completely out of German hands. Therefore, I don't think that WW1 stands as an example for how its possible to defeat Russia.
The Russian army brokedown because of a number of things. Events on the homefront were certainly important, but then so were developments at the front.
There are two points I'd make here.
First, is that the domestic situation of a country is incredibly important in warfare. Therefore ignoring it doesn't seem correct. Particularly when they are not isolated incidents. The USSR could reasonably be expected to suffer from many of the things the Tsar's government did; oppressed people, angry minorities, starvation, massive military defeat. That it didn't cripple the USSR in the Second World War is due to a number of factors; lend-lease, Stalin's use of discipline and propaganda, the brutality of the German invader. But that doesn't mean Russia and the Soviet Union might not have suffered from these again. Certainly, it's a factor to be considered, but the political situation alone cannot prevent the First World War being used as an example of some impossibility of invading Russia in modern times. Particularly once the Western Allies efficient logistics and strategic warfare methods are factored in.
Second, this idea that the Germans could only advance deep into Russia after the Russian army collapsed is rather incorrect. Going through the war; in 1914 we see the Russian army do rather well against the Austro-Hungarians, but against the Germans things are rather the opposite (this by itself is interesting because it rather dispels the common myth that the Russian army was the worst in Europe). By 1915 however, things really start to go down the pan. The Great Retreat really demonstrates how badly the Russians could fare when the Germans wanted to hit them hard. After that there isn't much offensive action (from the Central Powers) due to the pressure being felt on the Western Front. Certainly in this time period we see that the Russians had developed effective offensive art (particularly against the Austro-Hungarians, but the Germans were also successfully attacked) but that there was a failure to develop defensive art. The Germans could gain ground fairly easily from major offensives, when they weren't distracted in the West. This is before the morale collapse has truly happened. The Russian army only truly brokedown beginning in 1917 with the Kerensky Offensive.
As an aside, the Kerensky Offensive is by itself an interesting point. You had a similar situation in France with the mutinies, soldiers refusing to attack the enemy but accepting the need to defend the homeland. Essentially a strike more than a mutiny. The difference was that the French decided to press the "Less offensive, more wine!" button. The Russians meanwhile decided to press the "I'm sure they're just using reverse psychology!" button and ordered another offensive. Consequently French morale improved into 1918 and the army could again go on the offensive. The Russian army meanwhile collapsed.
The US invaded and controlled South Vietnam. They were unable to maintain that occupation for a price {money, human, and diplomatic} they were willing to pay.
Similarly, Imho, the USSR could not have successfully resisted an Allied invasion and takeover of control.
Beyond that the Allies had no history of or stomach for continued occupation of other countries {since the end of colonial empires} against concerted resistance.
So while the Allies could have defeated the USSR, I don't think they could or would want to occupy it.
The US did not invade South Vietnam.