Parking a domain on top of an addon domain in cPanel can be a quick, easy way to change the domain on a site without mussing with files or database connections and the like.
Yes, you can have a website hosted on one server, while having a subdomain of the same site hosted on a different server. Here’s a quick how-to tutorial for accomplishing that.
You can install regular SSL certificates to replace the self-signed copies that come with WHM (and therefore eliminate error warnings the customers receive using Webmail, cPanel, etc.).