In 1762 Catherine II (OTL Catherine the Great) comes to power in shady circumstances, after her husband Tsar Peter III is assassinated. A traditional Orthodox society frowns upon this but does little, except for a series of rebellions in the provinces which would eventually be compounded into, and fail as, Pugachev’s Rebellion.
Before Catherine or even Peter III had come to power, and Elizabeth (who had come before both) the Tsar had been an infant boy, Ivan VI. His mother was regent, and a statesman named Andrei Osterman, ran the country on his behalf, until he could come of age. But that day never came. Elizabeth, a daughter of Peter the Great, took power after a year, and the baby Ivan VI was sent to captivity. He grew up and died in prison-fortresses, although his death was caused by a botched attempt to spirit him out after a guard, named Vasily Mirovich found out about the young man’s heritage and claim to the throne-of-all-the-Russia’s (after all, he HAD actually been Tsar albeit for only 13 months), and sought to use him for personal gain, by freeing him and then hoping to earn a position of wealth and power, when he was restored to the throne. However Ivan’s guards had strict orders from Catherine herself, and when the prison went up in arms (Mirovich started a riot to try free Ivan), they promptly executed him. Mirovich’s objective was lost, his riot failed, and he was executed. So Ivan VI died in 1764, little more than just a footnote in Russia’s vast history.
At the same time as Ivan was meeting his fateful end, events elsewhere took place also regarding the throne and Russian rulers, also failed, but far more spectacular in scale.
Pugachev’s Rebellion (or the Cossack Rebellion) of 1773-75 was the principal revolt in a series of popular rebellions that took place in Russia after Catherine II seized power in 1762. It began as an organized insurrection of Ural Cossacks headed by Yemelyan Pugachev, a disaffected ex-lieutenant of the Russian Imperial army, against a background of profound peasant unrest and war with the Ottoman Empire. After the initial success, Pugachev assumed leadership of an alternative government in the name of the assassinated Tsar Peter III and proclaimed an end to serfdom. This organized leadership presented a challenge to the imperial administration of Catherine II.
The rebellion managed to consolidate support from various groups including the peasants, the Cossacks and Old Believers priesthood and its administration claimed, at one point, control over most of the territory between the Volga River and the Urals.
Government forces failed to respond effectively to the insurrection at first, partly due to logistic difficulties and a failure to appreciate its scale, but the revolt was crushed towards the end of 1774 by General Michelsohnen at Tsaritsyn. Pugachev was captured soon after and executed in Moscow in January 1775. Further reprisals against rebel areas were carried out by General Peter Panin.
While Ivan VI’s story and Pugachev’s happened at around the same time in history, they were not related except that they both centered on the Russian monarchy and the quest for achieving (and keeping) power (the Russian throne). In fact while Pugachev was searching for ways to legitimize his push for overthrowing Catherine, he was apparently not aware of even the existence (ever) of Ivan VI, who would have presented a much more believable person to claim to be, rather than the very-dead (and very-publicized) Peter III.
The main essence of the POD of this TL is that Ivan’s escape succeeds (his guards, even though with standing orders to execute him if he or anyone else made attempt to his escape, could have hesitated upon the cold blooded murder of an innocent young man, they may even have developed good relations with him during his captivity) and finds the steady talk of revolution and unrest (in the early 1770’s to be culminated in open rebellion), centering around Pugachev, to be a perfect tool to get the throne (back).