Not read all the thread yet.
I've seen a couple of posts about charity gigs, and tbh it looks like a nightmare .
If I were the op, I would say ' sorry , but due to illness we have to pull out of the event etc.'
I've always wanted to play a small gig , ( somewhere in the west end like borderline/100 club ). I would charge people on the door , with all the proceeds going to a small animal charity like the blue cross etc.
That means you just do your usual gig, without additional hassle / p@@s taking etc.