Why would the casket crush if it wasn't air sealed? If it wasn't air tight, then the air inside could be sucked out into space and then the pressure would be the same and the casket wouldn't crush. And Spock certainly wouldn't have been pulled out through a little hole a la alien newborn in Alien Resurrection.
And David clarified that the gravitational fields were in flux and it could have soft landed. And even if the landing did damage Spock's body, the Genesis effect would have restored him.
Force of the air being expelled, combined with an "easy-open" lid, wouldn't play out well, especially if pressure is the trigger to open the latch. Also, there would still be gases within Spock's body, which would be expelled as well.
In my head, I'm correlating all sorts of disastrous outcomes (which didn't happen in the movie) as a result of an improperly sealed casket being fired at extreme speed, out into the vacuum of space, and then into the atmosphere of a planet. Yet, the last shot of TWOK is Spock's un-tarnished and perfectly lying casket. It's not a stretch to say that the torpedo was made out of materials that are resistant to the effects of atmospheric entry, but then why would it "weather", and after such a short period of time too? (It can't have been more than a week or two between TWOK and TSFS)