When the guardian is away, who can make medical decisions?

Hopefully, the Guardian can be reached by phone if not available in person. If it's a matter of signing something, the healthcare facility will put 2 people on the phone at the same time to witness your instructions or decision.

I have signed documents for health proxy at the hospital on occasion. I believe it only lasts for the illness or injury at hand . You can put a coguardian on the person through an attorney. Once you get the court date it is usually granted and parties can either be present or just the attorney (however reasons for absence of any party is usually given). The decree will remove the primary guardian and regrant the primary (or another primary as case may be) along with a coguardian. I am coguardian to a family member along with another family member.

Don't know about Guardianship, but POA's and Wills (executors) should have an alternate or two to serve if the first person can't serve. I assume the court appointed the Guardian, so if the Guardian is gone, you may need to go back to court and get it changed.

I would think that the guardian of the individual would make arrangements prior to leaving the post. Instructions for the care for the family member would be managed accordingly with the arrangements set forth, of course in respect to the state guidelines.

One thing I wonder is about people who have guardians but are still competent. I have a friend whose sister is guardian. She took over guardianship when he had a serious stroke that left him blind for a long time. I wonder if, in a case like his, he would be able to sign for himself. I don't know.

I don't know if it is possible in most circumstances. A guardian is a bit like a parent of a child. Medical personnel can make decisions in emergency situations if the guardian can't be contacted. Otherwise they need to get the guardian to sign to do things that are out of the ordinary.