If I were to create an Archive forum, is there a way to copy particular postings into the Archive forum without closing or removing the post from its original forum?

Is there a way to protect specific postings from being purged? I know each post can be reviewed before purging but is there any other way to protect the post?

The normal per-topic move utility will close the original topic by default, but you can just reopen it after the copy is made.

The mass move utility in the control panel can copy topics without touching the original status.

At this time, there's no way to mark a topic as being immune from manual pruning. This feature will be present in the next generation product.