This is probably one of those times when my personal definiton for a thing doesn't match up with a common definition. Unless I am doinga very directed game (like for a con) "prep" NEVER means "decide what is going to happen" for me. it is just about laying out the situation, so that I have a solid foundation off which to improv coherently.
I hear you, but this isn't about the distinction between prepping situations vs. plots, ala the Alexandrian. It's more that you might come up with an intro scene or situation—often based solely on a couple details, in the moment, during the session—but no "if A happens, then X happens; if B, then Y." Like literally nothing past how it starts, then you flow from there. It's a matter of degree, which you can imagine being a meaningless distinction in the abstract, but once you start running it the difference is stark. The looser the framework, the better, otherwise you're more limited in your improvised consequences, and Devil's Bargains become less interesting and creative, etc.
To be more specific, the more tightly defined the situation is, the more likely you'll be to come with consequences that just resemble critical fumbles in a trad game. That's a huge and surprisingly common problem with starting out in PbtA or FitD. I've played with multiple GMs who default to fumbles without thinking, despite the fact that the rules explicitly warn against having consequences (on a partial success) negate the associated success.
There are 3 starter adventures in the book -- one for each ship. Although "adventure" is probably too strong a word. They are discrete scenarios. Weirdly, the smuggler one at least seems to suggest it should define the campaign going forward for a number of sessions. I don't think I would take that advice. S&V in particular just feels more episodic, with continuity emerging through interactions rather than some "ancient artifact plot."
You know, I was going through the book this morning, and I missed those yet again. I actually think SaV isn't presented as clearly as it could be, but that just sounds like me griping about my own lack of comprehension/memory.
Anyway, looking at those again, I still don't like em. And they're pretty clearly within the framework of the game's default setting, which I also don't like. I think they're as useful as a play example, basically.
As for the idea of a starting score establishing some overarching narrative elements, I don't think that's a bad idea at all. You're right that SaV is very episodic, but the game also says it should only really run for 12 to 20 sessions. That leaves plenty of time to let narrative thrulines emerge in play, but not a ton of time to just mess around. Coming up with some sort of metaplot elements early on doesn't hurt, or violate the game's principles. There could be a looming threat, but no defined sense of how the PCs might counter or even take advantage of it. You might turn a big bad in the first session into the central enemy for the campaign. The first session can absolutely set the rest of the game on a certain path. Or it can just be an intro. A lot of the advice in SaV is geared towards minimizing prep while setting up a narrative that doesn't feel totally random or meaningless. I don't like all of that advice, though.