Think they're arranged by last post By default. They probably should just stick in the order they're stuck though. Feature request maybe.
I think it needs to be changed by admin on settings.
There doesn't seem to be a way/plugin to copy a topic to another category. There is a way to move a topic if it's miscategorized, but it would also be useful to clone a topic if it fits multiple categories.
@bbb cloning topics is messy business. Do the replies get copied? Do later replies get copied? I suggest using tags if you have a lot of overlapping categories.
That's a policy decision. I can see at least two practical solutions:
Copy only the original post, and let the topic have a life of its own.
Copy only a reference to the topic, so it's in effect a different view of the same topic, and replies to any views show up in all views.
Tags don't quite address the problem.
@bbb If you move the topic from one category to another, you will have a "reference" to that topic in the first category. Probably if you move the topic more times, you will have more references in more categories.
@Giggiux Not sure what you mean. If you move a topic from one category to another, it disappears from the first category. I was suggesting the capability to have the same topic listed under multiple categories.