I suppose I can't really fault anything that Holmes does but the story as a whole does make him seem a little incompetent and gauche. Two different stories colliding.

I like your idea that Armstrong had been going through things already in his mind. Naturally he was upset, his patient was the wife of a friend. He would have had to be strong and supportive for Staunton's sake and there had been a lot of secrecy - it may have been a relief to be able to talk things through with someone else.

I hadn't even thought about children - yes, of course the young couple would probably have hoped for children in the future. They may have thought that giving up the future bequest would be selfish - they had to think of the next generation.

There is that explanation: The poor boy was half crazed with grief, and yet he had to go to London to play this match, for he could not get out of it without explanations which would expose his secret. But surely he would rather have known his wife was nearing her end, so he could be with her no matter what.