I wonder whether it would be useful to not only put the new one in writing, but have her read it alive for a couple of analog video camcorders along with, perhaps, a Super 8 camera. Although Super 8 is more than a little outdated and lacks sound (silent film is still readily available, but no undeveloped sound film exists that is in condition suitable for use) it is also likely to be harder to forge than most other media. Digital video can be forged with a desktop PC, but I don't know of any person or entity with the equipment necessary to forge a Super 8 camera reversal film. If someone had an unbroken strand of film that clearly showed Mae reading her will and holding up cue cards at appropriate times, and if the film precisely matched the action in a couple of videotapes that were shot from different angles, that would tend to suggest pretty strongly that the film and videos were all genuine.
Although Super 8 film isn't free, spending $75 for a camera and lights, $35 for film and processing to DVD (using the DVD as a viewing copy, but the film as the official copy) shouldn't break the bank.
And in seriousness, if I was her, I wouldn't cut Gaddy out entirely. If Gaddy has been a loyal help for the past 10 years, the moral thing to do is recognize that. Assuming Mae wakes up, I expect she's going to be a darned uncomfortable spot of trying to figure out which of her relatives is spinning the truth.