There's lots of good advice here.
I just wanted to start by saying that the story outline is interesting and should make for some very memorable scenes. Good luck!
When I'm designing an adventure, I usually have a small handful of "set piece" scenes like your castletop encounter with Dracula that I know I would like the lead the party towards, either because the encounter is tactically interesting or because they have a strong thematic element that supports the story I'm hoping to tell. Then I give the players absolute freedom in getting from scene A to scene B to scene C, or even do get to them out of order, while they explore, assemble clues, interact with NPCs, and figure out what's going on.
The key word in that paragraph is "hoping". If your players are on board with your campaign idea, you shouldn't have to force them to follow your clues and encounters. BUT, they may decide they don't trust the mysterious stranger, and once they realize he keeps showing up devise a plan to ambush them. Or they may figure out a way to capture Dracula as mentioned above, or just bring down the castle without ever meeting him. Or they might bypass the castle altogether and strike out into the wilderness (no way we're attacking Dracula, that dude is badass!) and reach the Lycan city first.
After long years of DMing, I've learned not to overplan. I generally know what the PCs are going to do over the next 2 sessions, but anything after that is totally up in the air.
The trick is to find a way to tell the story you want, while letting yourself be constrained by the player's choices. Feel free to design lots of NPCs that will be fun for you to play, but don't get attached to them--if the PCs or circumstances remove an NPC from the scene then you have to let it go without complaint. Design an encounter area and enemies, but don't force the PCs to approach it in a certain way or from a certain direction. They might stumble into it unawares, scout it and prepare an attack, arrive before the enemies and set up an ambush, or any number of things you can't think of ahead of time.
And if you have lots of time and desire to prepare ahead of time, or you don't want to improvise, then try to create an adventure with lots of choices: choose a door, choose to ally with this or that NPC, choose a strategy... it is possible to give players lots of meaningful choices and real agency in the game while still making them want to reach your cinematic moments.
