I've never figured out what is really supposed to happen when you shut off a worm-hole in mid-transit. In one episode of SG-1, some heavy material re-materializes inside of the nearby planet's sun (causing/solving the red sky and eminent doom). In another episode, Teal'c is trapped inside of the buffer, and his atoms are not just randomly lost at some point in space between the two gates. Also, there is at least one episode I can recall where a Jaffa retreating through a gate has his staff weapon cut in half when the gate shuts off. Also in the 2nd episode of the entire series of SG-1, Kawalsky had his head cut in half by them shutting down the gate while his head was partially in the wormhole. So the whole thing about transporting entire objects as one packet seems to be not true all of the time.
Can't believe I'm being this nerdy but everything you mention there is consistent in the show's canon
As you push things into the event horizon, they are dematerialised and stored in a buffer in the stargate - so if you stick the staff weapon (or your head) halfway in it's not "there" any more. Once the stargate decides the whole object is inside, it sends the data in the buffer to the other stargate via Sci Fi Awesomeness. It's sorta established that this is *not* instant. When the data gets there, the receiving stargate receives it into the buffer, and once the whole object is in the buffer, rematerialises it out of the event horizon.
So what happens when you shut the gate off depends what stage in this process you are at: if you shut off while a object is partly into the stargate then the bit in the stargate vanishes, no part of it was sent yet (the other half I guess is left in the buffer, but the buffer gets cleared when the gate connection *opens* at least). If you shut off while the 'signal' is in transit between the gates then you get the materialising in space scenario, which rematerialises it without its actual structure (just dumps the fundamental particles back out into 'reality'). Teal'c gets trapped in the buffer because the gate is malfunctioning and is refusing to rematerialise the objects it receives; they have to get him out before anyone else dials into the gate because this will clear the buffer and destroy his stored pattern.
So yah, it basically does transmit each object as a single "packet", but there is a buffering phase inside the stargate at each end to allow this, and the gates don't bother to push partially buffered objects back out if the connection is cut (guess the ancients weren't too big on safety).