A guest called me to confirm they would / would not attend. Can I accept (or decline) on their behalf?

Yes. Go to Guest List. To display all guests, including those who have not yet responded or registered, click the Guest Filter icon and select:

View All Invited

This will load all guests invited to the event.

Find the guest using the search bar.

Select their row using the checkbox. Then click the green thumbs-up button in the toolbar.

This will accept their invite on their behalf and set them as confirmed. You can also decline on behalf of a guest using the same method with the red thumbs-down button.