Mark, the Soviet leadership may not have publicly announced death sentences on foreign nationals (other than those already unfortunate enough to be within their captivity), but it certainly carried them out. As for not taking over embassies, they didn’t have to. They took the countries over instead.
On that latter topic, I’m not quite sure what that citation of the Montevideo Convention (never signed by Iran, incidentally, either pre or post 1979) is designed to show, but I do note that Mao’s Red Guards (like those Iranian ‘students’, they were proxies for the regime) stormed the British Embassy in Peking in 1967, set it alight and beat up and sexually assaulted members of the embassy’s staff.
Four years later, Mao’s gangster regime was admitted to the UN. One year after that Nixon was in China.