Well, there's no way any of them memorized the schematics or likely even had time to look at them, let alone figure out a weakness. Why not just send them off to whatever secret labor camps they have if that's the typical M.O.? I guess it's a moot point since it's never covered and didn't really happen. I just don't buy storylines where bad guys go to the trouble of taking someone prisoner just to go kill their prisoners somewhere else.
I don't think anyone memorized them either, but why chance that? You've sent Vader out to get them, personally, so I would think that alone is testament to how important the mission is... Being thorough is probably a good assumption then.
I think it could be very likely the droids are Rebels by the way, also being taken away to be "interrogated". Or scanned. Or whatever droids get.
As far as having time to look at them though, I've not really seen anything indicating that one way or the other. It could've been a very short period of time, or the pursuit could've taken quite a while with maybe different hyper jumps and the Destroyer only catches up to them in the Tatooine system. I know one source indicates the plans are picked up by a Rebel listening post that sort of examines them and sends them off quickly to the Tantive IV, and a pursuit is on from there then, but that it didn't seem to be an immediate thing where the Tantive IV themselves steal the plans from the Empire... Someone else gets them and transmits them. Who knows though, that's probably got 3 completely different stories to it.
It just seems most logical to me that the crew's taken, interrogated, and killed with a "cover-up" excuse so it looks like an accident... Plus killed so they don't ever possibly expose what happened. But also, interrogated because they may reveal other information too, that the Empire wants, beyond the DS's plans. They're still looking for the base and all.
But mostly to me, it goes to the idea that the galaxy's government at that time is more complex than simply Palpatine ruling everything and nobody having a say... They were covering it up for a reason. Leaving anything to chance seems sloppy on Vader's part who strikes me as simply a thorough guy.
Then again, this is the same organization that refuses to react as even a precautionary move when it's revealed that the Rebels have found something vulnerable, and are attempting to attack that something.