ICloud – Does Apple refund prorated cost to people who cancel their personal storage plan to join family plan before end of billing period

It's all detailed in the Apple Support Article: View and remove associated devices in iTunes:

When you use certain iTunes Store features on your iPhone, iPad, iPod touch, or computer, your iOS device or computer is automatically associated with your Apple ID in iTunes account settings.

and

You don't need to associate your devices to buy content, download new purchases, or to play back content that you've saved to listen to offline.

To remove a device:

Under the iTunes in the Cloud section, click Manage Devices. Click Remove next to the device that you want to remove.

Note that only computers are Authorized and Deauthorized, and are limited to 5 per account. See this other article: Deauthorize your computer using iTunes

Finally, this Discussion gave a good answer on Authorized vs Associated:

A computer can be AUTHORIZED to play/use content purchsed from multiple iTunes Accounts. There is a limit to the number of computers that can be AUTHORIZED per account (5 total) but no limit as to how many accounts a computer can be AUTHORIZED to play. Once you have AUTHORIZED all the accounts AND downloaded all the purchased content from each account to a computer, this content can be also be played on any device ASSOCIATED with the computer. A device can only be ASSOCIATED to one computer but one computer can have many devices ASSOCIATED to it. As an example, you can use apps purchased on from multiple iTunes accounts on an iPad as long as the computer the iPad is ASSOCIATED with is AUTHORIZED to use/play the content for each of the accounts. Another example: if you also purchased music from all the iTunes accounts, you could listen to it on an iPad, iPhone and iPod as long as all three were ASSOCIATED with the computer where all the accounts were AUTHORIZED.

Both FaceTime and FaceTime Audio are data and never go through the phone network as a regular phone call. You will be charged in the same way as using the internet for other purposes on your phone, over Wi-Fi or mobile data depending on your scenario.

FaceTime Audio calls don't ‘become’ regular calls. I'm almost certain the case from the linked article must be describing user error, calling someone not through FaceTime Audio but a normal phone call, and the user mistook the buttons they were pressing on their phone.

Besides any network fees, Apple does not charge a toll or fee to make or receive any calls. The license to the software and iCloud infrastructure is included with the cost of the Apple hardware.

There are certain countries that prohibit FaceTime services such as Saudi Arabia and UAE so you would not be able to make FaceTime or FaceTime Audio calls to these locations, but this would not cause a switch to normal calls. Net neutrality is evolving in several countries and there is no technical reason why FaceTime audio or video couldn’t be charged for above and beyond other forms of data.