What happens to objects that enter a Stargate from the destination side?

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.

If I recall, it tries to send it, but matter sent through the wrong side cannot reintegrate and is just a particle shower on the other side iirc. I think it was explained better in either the early episodes or a really late one.

It is really debatable. Taking into consideration how smart the ancients were, I don't think the screenwriters got this part right. The ancients would have put ALLOT of safety measures in place.

In countless episodes there has been stated that you must enter fully into the event horizon of the gate before you can reappear on the other side. So, I can draw the conclusion that the event horizon is part of the gate not of the wormhole. The wormhole ends at the gate, witch just materializes/dematerializes the information (people, objects) trough the event horizon.

I figure the logical safety measure would have been not to let anything pass trough the wrong way (if you try to enter the event horizon you would just press against a solid surface).

I know this is an old thread, but it is possible it is using what's know in the telecommunication industry as "Store and forward" which is a telecommunications technique in which information is sent to an intermediate station where it is kept and sent at a later time to the final destination or to another intermediate station. In this case similar to what they did in Atlantis, with the McKay/Carter Intergalactic Gate Bridge. But in this case it has nothing to forward too so the gate simply holds the "data". Again this is just speculation, and I am not a scientist, nor do I play one on TV.

I happen to be rewatching Season 1 at the moment. A lot of things change from first appearance, like zat'nik'tel disintegration. This is one of them. In Children of the Gods, Apophis and his group of Jaffa actually back right through the still-connected incoming event horizon, both on Earth and Abydos. It's highly likely they were making things up as they went along, especially in the early seasons. I'm making a list of 'rules' and noting discrepancies. I have three already: the CotG wormhole anomaly, Teal'c's growing knowledge of Goa'uld tech that he previously had no idea was more than "magic", and the current anomaly of the dominant Earth-Gate and the DHD - the SGC's Stargate still functions with the Antarctic Gate connected to its DHD.

So while at first it seems to be possible to re-enter and use a wormhole both ways, but eventually that changes.