I think, regardless of what he decides, he will be blamed for it. No one doubts that these two had a great relationship.
I agree. If he backs out of the Red Cross Ball, some people will accuse him of going to the funeral for the publicity. If he misses the funeral to honor his previous commitment to the Red Cross, those same people will call him insensitive. Also, as someone else here pointed out, he's 83 himself, and, despite the image the public still sees, maybe it's not that easy for him anymore to travel around on short notice. If Mr. Nimoy's family has a problem with William Shatner's decision, it's up to them to discuss it with him.
I have read no one from Star Trek liked Shatner. Have no idea if it is true.