1. Where is XenTag copyright line was displayed? Can I remove it?
It's displayed on Bottom of Tag pages & Tag cloud pages. You are not allow to remove it. If you want a Branding Free version, please send a conversation to me.
2. What is Auto Tag Linking feature?
It will look at what you tagged in your threads then find them in your post content, make these keyword into links (link to your tag pages). This feature increases your internal links and help Google or your members find your relevant content better.
There is an option for this feature (Auto Tag Mode), you can choose what you like there.
3. How can I disable Hashtag?
You can turn it off in Xentag options