History is a delicate thing, a complex interplay of action and reaction. There's more to alternate history than just deciding what you want to happen and ramrodding it without regard for the historical factors at play. What-ifs do not exercise themselves in a vacuum, but are subject to all the other factors that your point of divergence has no influence on. Your "what if" for this scenario was "what if Nicholas II is assassinated in February 1917". Wolfpaw has already correctly indicated what happens - Alexei ascends the throne with Grand Duke Michael and Empress Alexandra as regents.
This alone is a fascinating POD and could have sparked a nice multi-page discussion. In fact, I think that if you were to make a new thread with that as the opening post, you'd quickly get feedback that will help you begin to create a timeline. But there is no way at all that the Duma takes control of the situation that quickly, much less completely eliminates the monarchy's decision-making power.
What-Ifs are not license to decide what happens in an entire timeline. They modify the flow of history, and the joy and challenge of alternate history is figuring out how the flow will be altered. We know what would happen immediately after the death of Nicholas II, and the Duma calling democratic elections isn't it. Your timeline consists of you intervening to shape the flow of history the way you wish. You can't do that in alternate history, any more than you can alter the course of the Nile after throwing a rock in it.
Totally correct. Alternate history in the truest sense is not fiction. It's a intellectual discussion as to what would happen if an event or two were changed. When someone writes a timeline they are presenting they're interpretation of events in the way they believe is most accurate. Sometimes people agree, other times not so much. But any TL worth it's weight in gold is not that does everything the author would prefer. It does what the author believes would happen.