I do not think it is completely clear. As you suggest, in A Hundred Days they seem to indicate that the MALP is destroyed because it is falling back through the event horizon from the other side. Also, Teal'c goes to great lengths to secure himself above the event horizon to not fall back through upon arrival, leading us to believe falling back through would lead to his destruction/disintegration.
However, in the episode New Ground (Episode 3.19) we clearly see Nyan (the Bedrosian archaeologist) put his hand into the event horizon of the incoming wormhole from Earth with no ill effect. Maybe some small contact is okay, but attempting to completely enter the wormhole from the wrong side is not.
Taking the evidence from the two episodes above, I think we can hypothesize that touching or entering the event horizon of an incoming wormhole is not actually harmful. The Stargate itself must simply store the data from objects entering the event horizon in its buffer, regardless of whether the wormhole is incoming or outgoing. Since the Stargate only transmits discrete objects (and not pieces of objects), it does not attempt to forward the partial object and so the the object can easily be pulled back out. Once the entire object enters the wormhole the wrong way, however, the Stargate stores that data in its buffer and looks to send it on to the receiving gate. This is impossible if you've gone through the event horizon of an incoming wormhole since the Stargate cannot send the data on via an incoming wormhole, so the object is lost.
It is open to interpretation, I think, as to whether the object that entered through the event horizon of an incoming wormhole is still stored in the Stargate's buffer or not. If so, then once the incoming wormhole disengages, if someone wanted to retrieve the object, they could reconfigure the DHD to extract the data from the buffer (see, Episode 5.14, 48 Hours). However, the object would be lost if the Stargate is activated, clearing the buffer. It also may be lost after some amount of time (McKay has hypothesized that the buffer is only able to store data for 48 hours). (Once a new Stargate series launches, I'm totally pitching an episode in which the team hides in the buffer of a Stargate to elude an enemy.)
Of course, it is well known that if you touch the wormhole while it is being established, you are toast.