Correct me if I'm wrong, and since I haven't really read through CoS too much (skimmed through it a few times looking for individual things), but if you're already planning on having them meet him somewhere else, is there any reason you couldn't start the fight somewhere else? The reading is about the final battle with Strahd, right? It doesn't magically lock him in there, it just determines the choice Strahd makes at the time to have his last stand there, correct? If that's the case, can't you just rework it that the fight takes place across the entire castle with the finale occurring in the chapel?
So, they're going to meet him in the dining room, have a bit of social, and then be otherwise compelled to explore the castle? You could use leading encounters (have a creature flee to the next area you want them to explore type of thing) to drag them around the castle, hordes of undead to block off areas you don't want them going, things like that. 7 party members is a lot, is there any way you can force the party to split? You could then bounce each group around the castle in a series of weaker encounters (if you can split the group into groups of 3 and 4, or 2, 2, and 3, then you can always use skeletons, zombies, ghasts, and ghouls to drive the party around the castle without them feeling like you're trying to kill them by splitting them up, it would feel more like a haunted house ride) and have them have the goal of trying to meet back up. Again, I could be wrong, but isn't the castle itself a product of the mists, and so doesn't have to actually follow a logical progression? The mists creep in through open windows and from hidden chambers and whatnot, slowly filling the room as they fight weak monsters, they move to the next area and suddenly are somewhere entirely different. With a party of 7 I imagine you have at least 2 stealth characters right? Maybe you can find out what everyone's passive perceptions and investigate checks would be, and pull the ol' DM note trick to try and lure them away from the group, sending them on a stealth mission while the rest of the party assumes Strahd has done something to them and they search the castle for them.
Ultimately, it seems like you're trying to think of a solid reason to delay the final confrontation, and you want at least some of the encounter to range across the castle eventually ending in the chapel as the cards "predicted" (actually, dictate, but that's the metagame). Strahd can mist at will IIRC, where he effectively becomes immune to most damage and can then escape through cracks in the wall, open windows, chimneys, locked doors, etc. Couldn't you find a way to lead the party to the various event sites you're looking to run, and if it is reasonable that they would encounter and fight strahd at those moments, let them. He takes a round of hits and then mists and regens while he runs anywhere else in the castle. Skeletons and Zombies are gonna go down quick, eat up actions, can come pouring out of anywhere (they're undead, Strahd could have had them buried in the floors and walls and they burst out when he mentally commands them. 5th edition strahd is kind of weak, so a bit of DM fiat flare could be just the thing to spice him up for you (IIRC another poster before the forum crash had mentioned Strahd got one-rounded by a fairly low level party, you've got 7 PCs who could potentially nova the crap out of him).
I would suggest improvising some interesting ways to separate the party, and move them to the places you want around the castle with weak enemies, blocking areas with hordes, and having some more intelligent creatures who would know to run when outnumbered to drag the party behind them. I don't think anything, in this instance, would be wrong with you throwing out a dozen or so zombies and skeletons with a couple tougher creatures (maybe a vampire spawn or something, a solo creature that can take a few swings before it dies, living long enough to pull the players into another area) at the main group, and as they wade through the undead, you pass notes to players asking them to make some saves/checks, if they fail they fall through a pit trap through the mists and awaken in another part of the castle. Chase them back together, have them confront strahd, he flees to areas they haven't been yet that you want them to go to, and then finally to the chapel for the final curtain.
Again, this could all be based on faulty assumptions, so, take it with a grain of salt. I didn't think there was really all that much to do in the castle other than some other fights, so maybe you don't want to split them too much, but leading encounters, shifting the castle around, and gloat-and-float tactics from Strahd could let you burn through the party's resources before the big fight and make it at least a little bit more challenging than he would otherwise be against a 7 character strong party.
*predicted was predicated... -_-