Surely we could ( I say "we", meaning faders ) have some legal document that trumps the WTBTS's document? If you really feel that you must sign it for fear of being DF'd, which I understand, you could have a lawyer help you make another document that nulls the WTBTS death warrant.
The Eldubs could bring their copy to the hospital, only to be told that they have zero right to interfere with your medical wishes and that they are not entitled to your medical history. The Doctor could say, " Get the F*** out.."
Let's be honest, the WTBTS document is for Watchtards. They want the faithful sheep as willing test subjects for bloodless medicine, which they probably own stock in. If the WTBTS can have a "medical directive" for you to sign that trumps all previous documents, then a lawyer can make one for you that trumps the WTBTS's directive.
I'm no legal expert, but I didn't see any words to indicate that if you signed the WTBTS document, you couldn't change your mind.