Yes, it’s easy. Go to this page, purchase the premium version and install it into your theme. Once you activate the premium version, the free version will automatically be deactivated.
No, the premium version will keep your settings besides unlocking new features.
The problem might be concerning an incompatibility between the payment method chosen and the currency set in your order. For example, pounds are not supported by Authorize.net.
No, this possibility is not conceived. Users will be able to pay using any card within Authorize.net circuits (Visa, Mastercard, American Express, Discover, Diner’s Club, JCB). You may lead users to think there is a limitation concerning credit card systems available, by displaying only logos of some of them.
If you are using the free version of the plugin, this is the standard view. Yet, if you use the premium version of the plugin, you will be able to send the detail of products purchased to Authorize.net via API. To have this work, make sure you select the option Enable itemized transaction in the plugin settings. You can read more about this feature here.