An old post of mine:
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Why I doubt this had any real chance of happening:
(1) It seems to have been very much a minority view among the students and (according to Bowden) was only raised by--at most--two students who opposed the US embassy occupation in the early stage of planning. The original stimulus of the occupation was after all the US allegedly plotting to restore the Shah:
"The plan was the brainchild of three young men, Ibrahim Asgharzadeh, an engineering student from Stead Sharif University, Mohsen Mirdamadi from Amir Kabir University, and Habibullah Bitaraf from Technical University. Asgharzadeh was the first to suggest it They would storm the hated U.S. embassy, a symbol of Western imperial domination of Iran, occupy it for three days, and from it issue a series of communiques that would explain Iran's grievances against America, beginning with the overthrow of Mohammed Mossadeq in 1953 and decades of support for the shah, now a wanted man in Iran accused of looting the nation's treasure and torturing and killing thousands. America's imperialist designs had not ended when the shah fled Iran the previous February. The criminal tyrant had recently been allowed to fly to America on the pretense of need-ing medical treatment and was being sheltered there with his stolen fortune. America was stirring up political opposition to the imam, instigating ethnic uprisings in the various enclaves that made up the border regions of their country, and had recently begun secretly collaborating with the provisional government to undermine the revolution. A clandestine meeting in Algiers between secular members of the provisional government and White House National Security Adviser Zbigniew Brzezinski had been revealed to dramatic effect in Tehran. All of it added up to only one thing in the students' eyes: America was determined to hang on to its colony and restore the shah to his throne. The danger was pressing. The provisional government had sold out; it was nothing more than a group of old men wedded to Western decadence bent on tamping down the ardor of the Islamist uprising. One thing the revolution had taught the students was the folly of waiting for something to happen. They had seen the fruits of bold, direct action. Seizing the embassy would stop the American plot in its tracks and would force the provisional government to show its hand. Any move against the heroic embassy occupiers would expose acting Prime Minister Mehdi Bazargan and his administration as American stooges. The students believed that if they did not act soon to expose him if his govern-ment weathered its first year, then the United States would have its hooks back in Iran for good, and their dream of sweeping, truly revolutionary change would die.
"When Asgharzadeh had proposed the move [seizure of the US embassy] two weeks earlier at a meeting of an umbrella activist group called Strengthen the Unity, it was opposed by two students, Mahmoud Ahmadinejad from Tarbiat Modarres University and Mohammed Ali Seyyedinejad from Elm-o-Sanat University. Both preferred targeting the Soviet embassy instead. Asgharzadeh, Mirdamadi, and Bitaraf voted them down and then had expanded their planning cell by inviting activists from various local schools, including Hashemi, Abbas Abdi, Reza Siafullahi, and Mohammad Naimipoor, all young men experienced with street demonstrations and organizing. These Brethren were both students and members of the fledgling intelligence services. All of these men, including Ahmadinejad and Seyyedinejad, eventually joined ranks behind the seizure of the American embassy...."
https://books.google.com/books?id=5m0kyPc18l4C&pg=PA9
So the whole motive of embassy-occupation was clearly tied to US, not Soviet actions. For all its atheism, the Soviet Union could not be the focus of anger the way the US was; it had not restored the Shah in 1953, it had not provided him decades worth of support, and had not recently admitted him. (Of course its conduct in Afghanistan was disturbing, but remember that the Soviet invasion did not take place until a few months later.) The idea of occupying the Soviet embassy was at most proposed by two students who opposed the US embassy occupation. (That one of them later became president of Iran is of course true, but says little about his power in 1979...)
(2) But did even Seyyedinejad and Ahmadinejad propose to occupy the Soviet embassy? Seyyedinejad at least was later to claim that his own opposition (as well as Ahmadinejad's) to occupying the US embassy was based simply on Khomeini having warned against lawless acts against symbols of the old regime. And without exactly confirming or denying Ahmadinejad's advocacy of a Soviet embassy occupation, Seyyedinejad suggests that Ahmadinejad's militantly anti-Soviet attitude was unusual among the student movement as a whole and a product of the right-wing atmosphere at Elm-o-Sanat University:
"Several sources, including the former security official Hajjarian, said Ahmadinejad believed then that the Soviet embassy, not the US embassy, should be taken. ‘Ahmadinejad believed that the great Satan is the Soviet Union and that America was the smaller Satan,’ Hajjarian said. The other student leader who, along with Ahmadinejad, had opposed the takeover of the US embassy in the initial planning meeting of the student leaders was a young man known as Mohammad Ali Seyyedinejad.22 Years later in a newspaper interview Seyyedinejad said that he and Ahmadinejad had opposed the plan on the grounds that Ayatollah Khomeini had banned lawless actions against targets that were deemed to be connected to the previous regime. ‘A little earlier, the Imam had said all attacks against the offices and assets of capitalists, like hotels, had to stop. And if there had to be an expropriation [of an asset], it had to be done according to the law,’ he said, almost a year before Ahmadinejad had taken over as president. ‘In that meeting we told others that these kinds of actions were being carried out by groups who were in opposition to the government, and we should not act in a way that would confuse us with the opposition groups.’ 23 As for whether Ahmadinejad was in favour of storming the Soviet embassy, as had been suggested, Seyyedinejad said, ‘You must have in mind that Ahmadinejad was at the Elm-o-Sanat University which was generally very right-wing and anti-left … and anti Soviet.’" Kasra Naji,
Ahmadinejad: The Secret History of Iran's Radical Leader. https://epdf.pub/ahmadinejad-the-se...der3f87d25f1cce96a24de1b1d0fea1e63247916.html
For confirmation of the unusually right-wing nature of the Elm-o-sant University students and of Ahmadinejad's initial objections to the US embassy occupation (whether or not he advocated occupying the Soviet embassy) see Michael Axworthy,
Revolutionary Iran: A History of the Islamic Republic, p. 371:
"It was suggested after he became president that Ahmadinejad was one of the students involved in the hostage crisis (and some former hostages even identified him from photographs), but in fact, although he was one of the most forthright student activists at his university at that time, and had dealings with wider student organizations involved in revolutionary activities, Elm-o-Sanat was the only one of the five Tehran universities that did not participate in the hostage-taking. Being more inclined to the religious right than the left-inclined students at the other universities, the students of Elm-o-Sanat, like Ahmadinejad, tended to come from humbler backgrounds. Some have suggested that Ahmadinejad was more in favour of occupying the Soviet embassy, seeing the communist Russians as a greater threat than America; alternatively that he took seriously an injunction from Khomeini not to take part in provocative actions. It seems quite likely that once the occupation of the embassy began, he regretted that he had not got involved, but the students in the embassy were quite jealous of their prize at the time and wary of outside involvement in any case...."
https://books.google.com/books?id=XYoeAAAAQBAJ&pg=PA371
(In fact, I wonder if Ahmadinejad, if he did advocate occupying the Soviet embassy. did so not because he seriously expected it to be adopted by the students from other universities, but as a sort of reductio ad absurdam of the occupation of the US embassy. "well, if we're going to occupy embassies, why not the Soviet one?")
(3) Anyway, the most I can see is some right-wing students at Elm-o-Sanat (including Ahmadinejad?) trying to occupy the Soviet embassy. But even if they succeeded in doing so, I am pretty sure Khomeini would quickly order them to leave, and seeing their isolation, they would agree. Khomeini's immediate concern was with the US, and he shared the general view of the students at the other universities that the provisional government was too friendly to the West, so he seized on the US embassy occupation as a chance to remove allegedly pro-western officials. There was no similar motivation for him to move at once against the Soviets. Yes, he strongly disliked atheistic Communism, yes he would later brutally repress the Tudeh (Communist) party but that was not his immediate priority, and was simply not worth the risks and complications.