Is anyone other than the person who made a living Will allowed to sign it or make any changes to it?

In Florida, only the person who made the Living Will can sign it or make changes to it, unless that person cannot sign for himself or herself.  If someone wishes to make a Living Will but cannot physically sign their name, they can make a mark and have it witnessed.  I routinely have two witnesses sign Living Wills and a notary.

A person who lacks capacity cannot make a Living Will.  Capacity depends on the circumstances.  If a person understands his or her health and is capable of making choices for health care, they likely have capacity to make a Living Will.  Once a person loses capacity, another cannot make a Living Will for them or alter the existing Living Will. 

A Living Will is not required in Florida, and I doubt it is required in any other state.  If a person has no Living Will, state law says who may make decisions for someone who lacks capacity.  This is usually a spouse or close family member.  In most states and Florida, the "surrogate," whether formally appointed by a surrogate designation form or not, has a duty to make the choice that the patient would have made.  If the surrogate does not know what the patient would have chosen, he or she makes a choice that is in the best interests of the patient.

I have said more than you asked.  I hope this is helpful.