Good evening, my dear old chum Douglas!
First of all, please don’t worry about the game, the feedback, or the review. They’ll all be here whenever you have the time. Some things in life are far more important than a few lines of code and a handful of Mahjong tiles. 
I must say, though, I really admire what you’re doing. At 76, taking on a dozen songs, learning guitar parts, and singing for the first time — all while dealing with fingers that aren’t quite cooperating — that’s no small challenge!
I’m sorry to hear about the trouble with your hand. I had a little look into the ulnar nerve issue you mentioned, and I can imagine how frustrating it must be, especially for someone who loves making music. Please take good care of yourself, and don’t push those fingers too hard.
What touched me most was that this concert has a special personal meaning for you. I don’t know the story behind it, but I sincerely hope it becomes a beautiful and memorable occasion.
And remember, you’ve been a drummer for decades. You’ve already got the rhythm in your bones! The guitar and singing just need to catch up. 
I’m glad you’ve got good friends rehearsing with you. Music, friendship, and a reason to get together — there’s something rather wonderful about that.
I wish I could be there to hear you perform. If someone happens to record a little of the concert, I’d genuinely love to see it someday.
Until then, take care of yourself, enjoy the rehearsals, and give that concert everything you’ve got — without sacrificing your fingers in the process!
Warmest wishes from your old friend in China,
OldYoung 
