"Ahhhh, not so wounded as we were led to believe!"
Not "meant" but "led." In other words, he and the others believed what Spock wanted them to.
But that's what I mean. Khan knows he was being led; it doesn't follow he allowed himself to be led. To the contrary, it might follow that he knew all along that he was being led. His actions
have not been visibly affected by the coded exchange: he simply conducts his repairs and immediately launches into action at the completion of those. Surely he'd do that regardless of whether he believed Kirk and Spock or not.
it stands to reason that they'd be scanning ahead to get an overview of any system they neared, especially if it were largely uncharted
Yet it is a common feature of Star Trek before and after this movie that starships do not do such things. Spock only scans for the dramatically interesting after
Kirk specifically asks him for this exceptional action, or a sidekick character indicates confusion. Take for example "The Doomsday Machine", where the heroes get "within" a star system in their search for a distress call and only then
does Spock point out that there's rubble around them. Sulu was at a loss to explain his difficulties in locating... whatever, before Spock did his scan.
This actually makes sense for starships that perform quick in-and-out missions to dozens of star systems per month. Surveying is for wimps, and a planet never suddenly bumped into anybody despite not having been scanned (at least unless Trelane was steering it).
Heck, at the very least they'd want to scan for navigation hazards like, say, a huge cloud of debris from an exploding planet.
There didn't seem to be any hazard to the Enterprise
from the rubble of multiple planeticide in "The Doomsday Machine", either before or after our heroes realized they would be facing such a phenomenon.
Probably the only true hazards to starship navigation are exceptionally energetic phenomena such as tri-isophasic subspace manifoldatrons, and there's a passive sensor rigged for detecting those.