1. The userfix script works with any integration. It grabs the userid of a user based off the username of a user. Like stated the usernames need to exist in your forum. it is the same script and is in the photopost import forum
2. well if you want logins to be write cookies need to be correct they need to be integrated correctly meaning also your xenforo global salt needs to be correctly entered where asked on install.
3. You need to reset any group permissions after doing what has been said above. This is why you need to delete any entries in the usergroups table and then reinstall to integrate with xenforo.
4. I have never heard anything about this so without seeing a physical url I can not comment.