FWIW: (This is as of about 4 years ago, so anything more recent supercedes what follows (or someone with a more concise/accurate memory HAH))
If the appeal committee agrees with the original group, the DF'ng is immediately announced. End of story.
If the appeal committee disagrees with the original decision, and upon consulting with the original committee gets them to agree that there shouldn't have been a disfellowshipping, no announcement is made and the matter ends (except that they might feel they need to do other non-DFng stuff like restrictions, etc.) End of story.
If the two committees disagree, they send the thing into the branch for review, and no announcement is made until they get further along. Never had first hand knowledge of this particular flavor, but doubt the Society would be quoted as doing the disfellowshipping in the sense of choosing which committee to go with.
In the appeals situations I was personally involved in as an elder (both as part of an original committee and sitting as part of an appeal committee) never saw one overturned, but I've heard of it. The Flock Book and other training for elders makes the point that if things could go "either way", the appeal committee would likely side with the original decision, (particularly if the issue revolved around "repentance" or "lack of repentance") since they heard the whole thing "fresh", etc. etc.
Hope this helps.
---Dan