Disabling iPad ringing when someone calls an iPhone in the same account.

You could disable stop the calls from going to both devices by turn the settings below to off:

On your iPhone

        Settings ➡ Phone ➡ Calls on Other Devices, turn off Allow Calls on Other Devices

On your iPad

        Settings ➡ FaceTime, turn off Calls from iPhone

https://www.experts-exchange.com/questions/29195361/Stop-iPad-from-ringing-when-my-phone-rings.html

Would my wife still be able to do Facetime with our family on the iPhone and the iPad?

You need to double check on your setup for Facetime for your iPhone and iPads to check whether you have set a different email address in your iPad from the email address in your iPhone.

Goto Settings - > Facetime for each iDevices and make sure all of them are using an unique email address each.

https://www.macrumors.com/how-to/use-facetime-email-address-instead-of-phone-number/

???

Sounds like the answer is no then.

Another thing where Apple in their infinite wisdom knows how people want their devices to behave better than the individuals.  Sas but there's no need to merge telephony with videoconferencing or at least it sgould be a user choice.

Why you say the answer is NO?

It is your choice.

You can choose to do Facetime using email address instead of a telephone number.

Just make sure you have added additional email addresses in your Apple ID and each email address is assigned to one iDevice manually.

https://support.apple.com/en-us/HT201356

My point is that we have a single account for all these devices which allows us to get to all the same music, apps, subs, etc.

We're not going to create new accounts (and new email addresses) for each device.  You said we needed to have different email addresses for each device.

I think the key was disassociating the iPad with the phone numbers.