The fact is that the existing statutes would provide adequate protection, given good judges. And given bad judges, no statute could provide adequate protection.
Suppose, for example, that a statute required any "right-to-die" wishes to be written down, but Michael Schiavo offered the court such a wish, "found in Terri's bedside table", and signed with her name, but in handwriting that suspiciously resembled Michael's. If a trial-court judge wanted to accept such a statement, based upon Michael's testimony that it was found in Terri's bedside table (allowing, but ignoring, testimony from handwriting experts claiming the note was fake), Michael's statement would be sufficient to have her killed. So what would the "written statement" requirement accomplish?
Fundamentally, trying to write legislation to replace judicial common sense is a dangerous exercise which will seldom have the desired effect. Far better to appoint judges with some common sense.
Of course, now common sense is considered a disqualifying factor for judges...