To piggyback on this a bit, why was Leningrad's name reverted to its pre-USSR name, while Stalingrad was changed to Volgograd rather than reverting to Tsaritsyn?

Because it wasn't Russian before, it was Köningsburg of German East Prussia. The city was almost completely destroyed in the war, and virtually the entire ethnic German population was evacuated at the end of the war, and subsequently replaced by Russians. So there was/is little historical feeling and connection to the old names among the population. In the case of Kaliningrad Oblast, everything was renamed as part of the Russification of the area (e.g. Gusev, formerly Gumbinnen, Baltiysk formerly Pillau).

But even in Russia proper, so to speak, many places kept names for Old Bolsheviks; There's both Kirov and Kirov Oblast, for instance. There's even Dzerzhinsk named after Felix Dzherzhinsky, founder of the Cheka and a rather horrible person, to say the least. Leningrad the city is now Saint Petersburg but still surrounded by Leningrad Oblast. To some extent it's a matter of 'branding'. Not so many people lived in Kirov at the time it was renamed compared to now (70k vs 500k) and it only became a town in the 19th century. By comparison if you look at something like Nizhny Novgorod (named for Maxim Gorky during the Soviet period), that's a very old city and historically quite important, so it's name was very well-worn.