But why would it have an inner latch in the first place? It's unnecessary, given that only the inanimate equipment and raw materials will be inside the casing. As for why they would remove the latch mechanism, you answered your own question - namely that it was going to be used as a burial coffin. Burial coffins are usually sealed shut with nails, etc before they are buried underground. In the case of Spock, it was important to protect the crew from the radioactive fallout that his corpse would be emitting. I'm not sure anyone would have turned up for the funeral service if there was a chance they'd get sick with radiation poisoning! With that in mind, It's quite possible that they welded it shut (using a laser torch of phaser), and that the metallic inner is made out of lead. Regardless of what the movie novelization for TWOK/TSFS suggests, or the confusion of David and Saavik on the Grissom, TWOK movie clearly stresses that the intention was to deliver Spock's body to the Genesis planet - intact presumably.