I KNOW you did what you should have. Honestly, you don't know why that card was not signed... Your mom might not have signed it because she didn't want to or because she was undecided or torn. It was HER responsibility to sign it. To take that liberty and forge a name would have been morally wrong. What if she knows as a JW that she should say no to blood, but she hadn't fully decided whether to sign the card or not? On top of that, she is instructed to have it on her at all times. This makes me think it's even more likely that she may have had doubts. If you had immorally forged her signature and she died, you would have chosen something for her that only she had the right to decide.
That elder was morally and criminally WRONG.
Welcome to the site, and please think about this from a coldly logical stand point. It was her responsibility before her god to prepare that card. NOT YOURS. You did right.