If you delete all the files in public_html folder then it is as new as fresh account creation.
if you use frontpage then don't delete the files and folders starts with" _vti" in public_html folder.
To backup goto fielmanager navigate to public_html folder
Then select all and use compress to make an archive.
Then download the archive to your computer.
For mysql, goto phpmyadmin and use export feature to save the database
Yes you can use .com domain on x10 account.
To point the domain
You have to set name servers of the domain to
ns1.x10hosting.com
ns2.x10hosting.com
The propagation of DNS may take upto 48 hours.
Then change the primary domain in your account panel
OR
park the domain in...