Personally I would say that of all the leaders, Gorbachev had it the easiest and had a situation that was much more manageable. His situation was the most salvageable of all the four leaders IMHO.
Here I will grade difficulty of situation of each leader (10=most difficult).
1) Louis XVI: Difficulty = 7/10
He faced a situation that few if any monarchs had ever faced. With economy in ruins mostly due to his predecessor and an intransigent nobility unwilling to countenance any reforms required a very exceptional leader to manage.
2) Cixi: Diffculty = 10+/10
Cixi's situation was the most difficult of all four. She faced the same intransigence to reform from an ossified and corrupt gentry similar to Louis. But unlike Louis she also had to face off multiple world powers encroaching on China at the same time and in a world rapidly advancing. She was in an insurmountable situation IMHO.
3) Nicholas II: Difficulty = 6/10
Although Nicholas faced challenges, Tsarist Russia was in a much better shape, relatively, than China. It was technologically behind the other powers but much less so than China and its nobility was practically begging for reforms (unlike with Louis and Cixi). His disastrous forays into wars of his choosing also made things more difficult than needed.
4) Gorbachev: Difficulty = 4/10
Of all the leaders, Gorbachev's USSR was in the strongest situation. His country was a global superpower with tech only slightly below that of the West. By the time of his rise, USSR possessed nukes, enough to destroy the entire world. His state was completely secure from any external threats. Some economical reforms to loosen up state control and giving up trying to out compete the West militarily while maintaining authoritarian rule was all that was needed. A more competent and savvy leader, like Putin, could have maintained a superpower USSR to this very day.
In summary, it goes (least to most difficult with difficulty rating out of 10):
1) Gorbachev: 4/10
2) Nicholas: 6/10
3) Louis: 7/10
4) Cixi: 10+/10
Do others agree with this assessment of level of difficulty as well?
Nicholas should be about a half a point- all he has to do is make a few minor adjustments to his military or foreign policy and he's there. In fact he could easily have gone down as Nicholas the Great if he quickly crushes Japan and uses the resources for a quick win in WWI. Very plausible PODs there
Cixi- she faces a horridly complex internal situation. She understands that reforms are needed but can't fathom what they would be. Internally, the Manchus are very weak and need to keep the Chinese in check. Some obvious reforms would be impossible as they would raise the Chinese relative position against the Manchus.
Gorbachev- he's much weaker here than you think. He can do a lot by moderating his foreign and defense policies and relying on the nuclear deterrent. The problems he faces though are severe-
1) dissatisfaction is much more widespread in the Soviet Union than any of the other societies
2) their is the ethnic tensions within the Soviet Union. Decentralization is needed but to decentralize the country is to encourage the forces of dissolution
3) his greatest weakness is the Eastern Europe buffer. These states are not under his direct control, seethe under Soviet occupation and wish to throw off the foreign yoke. Its the Hungarian reforms that spell the end to the Soviet Empire
That said, yes, a true authoritarian could keep the regime in place for decades