But my answer to the opening post and my feeling is that if Lenin had failed, if the Russian Revolution had failed, Nicholas would have remained on the Russian Throne for the rest of his life which could have lasted into the early 1950's.
The February Revolution succeeds--hence the mentions of Karensky and the "return" of the Romanovs to the throne. The old succession laws were thrown out when the position of the Tsar was dissolved by Karensky and the provisional government, and truth be told, Nicholas was not ready for the throne which explains his overly-protective nature (the family was his escape), his bungling in handling WWI (he was never taught how to fight a war), and his general feeling of ineptitude (he had big shoes to fill after Alexander III). This is why, after the Romanovs are returned ITTL, that Olga is handed the crown. Nicholas is unfit, Alexei is a liability in a rebellious Russia (remember--the Tsars liked to tour the battle grounds and Alexei was a hemophiliac. Plus, victorian medicine said stress could bring on hemophiliac attacks, so the stress of ruling a nation would probably have been more than Alexei could bear according to medical texts of the time), Tatiana is too interested in growing up to be like her mother, and Anastasia is still just a child. The only "direct" heir would be Olga in this sense. In the revised timeline I'm working on and will be posting later on, the brothers of Nicholas have been killed and so are out of the picture.
I've taken a lot of the comments you have all offered and the new timeline is radically different, with Olga and her husband coming to power under a military coup and ruling like the Perons, and a 9-year Russian civil war preceding this event, Western European civil wars bringing Communism to bear in France and Germany (Independent Bavaria! Woo!) and the creation of la Grandiose République or Grand Commonwealth (literally, Grand Republic)--a pseudo Western-European USSR centered in Paris.
I hope the new timeline (when I finish it) is more palatable than my blunders above.