When you import, you should drop your threads database and run the createtables.php file in your install directory then run the importer file.
When you import, your data will come (threads, posts, pms, etc); your "settings" are incompatable, as are your styles. This question has been asked hundreds of times if you use the search feature.
Your custom fields (the standard 4) will come over. If you added more through a mod, they will not.
They will be imported over.. Rather they should, i can't see why they wouldn't.
for #5 see my response as #1
