@RonnaS, I'm a member on the Paul Godfrey Corfield music Discord.
On that discord, there is a person who is VERY involved with the gathering and cataloging of
Tolkien's letters, including many (hundreds and more) not yet published. He and his team work directly with the estate I believe. (And all they have is catalogued into a searchable library for Tolkien nerds.)
I have a suspicion he may be able to give you some clear insight into the letter, the circumstances at the time, and its existence/validity. Also, even of it's not directly from Tolkien himself, it's most likely from someone authorized to write on his behalf during his last days.
While it may not bear the same lustre, it should, in my personal opinion, still have just a little shine to it for you.
If you're interested, I'm sure he wouldn't mind taking a look.