Hiya!
First, congats on taking up the reigns of DM....it's not something many folk do well nowadays.
Second, and this is the disheartening part, I think you have much to much of a "railroad story" going on. You have a story. You have a beginning, a middle, and an end. This is great for a novel or movie...but not so much for an RPG adventure. Someone wiser than me once said,
No DM-laid plan survives the first encounter with the PC's, or something like that. For example, what if, in stead of "accepting and coming to terms" with them being infected...they focus on curing themselves? They are LG after all...they are HIGHLY unlikely to just roll over and accept their new evil-lords as "the good guys". I can see them fleeing the area to seek out cures. Or, I can even see characters who are *really* concerned about their condition actually offing themselves. A truly Lawful and Good person would rather die than risk himself/herself killing an entire family of eight, for example.
The releasing of Dracula is one of those pivoting points. Players are a crafty bunch (sometimes annoyingly so!). What if one of them figures out what's going on and actively prevents him from being released? Or, if they fail, what if someone comes up with some oddball plan that would actually 'capture' him? The Dice Gods are fickle...and if you say "Ok, fine...roll a 20"...and then he does roll a 20. What then? You can't just say "No. forget it. I take that back. He gets away". Because if you do, your players will then do everything in their power to "wreck" your story line; from their perspective, it's only fair...you 'wrecked' theirs by going back on what you said. And, after all, it *is* supposed to be
their story, is it not?
The castles/keeps thing is one of those things that changes the entire dynamic of the campaign. PC's go from worrying about which inn serves the best ale, vs the one that has the most comfortable beds....and start worrying about the growing peasant complaints about sewage in the streets due to lack of rains, and if the grain growers guild is going to want to sell more of it's product "out-of-state" to get a better price?
Lastly, don't do the "now the mysterious figure is my DM character!". Trust me. Just. Don't. Do it! If there one thing that is universally hated by all players I've ever come across (and thats 34+ years of actual gaming), it's the "Mary Sue" DM pet character. Especially one that has been (effectively) "immune to plot developments" for the last multiple game sessions. Just keep him as a regular old NPC. Oh, and I *highly* suggest that he not be portrayed as "the terribly mysterious shadow figure that has been leading you around, 'helping' you"...players hate that even more. If they ever find out that "all their choices" weren't actually their choices, or that all the "things that happened to us" happened because "The Sphinx", who is terribly mysterious, has been pulling strings behind the scenes to make them happen...well, lets just say your campaign will not end pretty.
Suggestion: DON'T focus on the PC's. Yeah, I know it sounds backwards, and there are many DM's who will disagree with me, but by focusing on the PC's, you are actually depriving the players of the freedom to play their characters they way they want. In your story, and that's what it is, you make all these assumptions that the PC's will do this, accept that, behave in this way or that, then choose option #38. If you write stuff in, or play it out, in order to make all these "plot points" happen, it will become obvious to your players early on... and then it's not a RPG. Now it's barely a choose your own adventure. Closer to a made-for-TV movie script that the players "get" to read through and say the lines.
Sorry to be so harsh there...but grand story lines
never play out the way the writer of the adventure had intended. Why do you think many of the earlier Basic D&D and 1e AD&D adventures are so well-loved? Because they had the foundation of the story going on 'in the background', but it was up to the players to create the actual story. Minor story sub-plots did go on in the background, to support the overall "story", but no plot/story "lines" were really there or so integral to the adventure. For example,
Keep on the Borderlands; no "plot line", but the story was basically "Here's an out of the way keep that has been plagued by raids from humanoids for a while". That's it. What humanoids? Up to the players and DM to discover. How long? Up to the DM. Why? Up to the DM. How do the PC's stop it? Up to the players and DM. So the actual "story" that takes place in, around and during that adventure is almost entirely created by the players as they play, and the DM as the players start to get excited about certain things (e.g., if they really start to see the lizard men in the swamp as the ones behind it all...the DM can run with it and focus on that).
Good luck no matter what you do! DM'ing is, IMHO, the best, and I prefer it to playing 9/10 times. I just like to freedom to create and show to my friends...and watch them explore what I created. Nothing like it, really.
^_^
Paul L. Ming