What’s the difference between therefore, anyhow and however?

In the sentence in red, the given answer is anyhow, but I feel therefore is the best. Can you tell me which is the best? And what’s the difference between therefore, anyhow and however?

This year I decided to do something to regain my reputation as a kindly uncle. My nephew, Tony, had never forgiven me for the dictionary I had bought him as a birthday present last year. His parents had no reason to be grateful to me either, because the year before, I had presented their dear son with a pot of paste and some funny pictures. Instead of sticking them into a book, Tony had naturally covered every wall in the house with them. This year, _____, I decided to let him choose for himself.

We went into a big toy shop but Tony was highly critical of everything he saw. In vain did I show him toy after toy; he was not to be tempted. Then I saw his eyes light up: he had discovered something he really did approve of: a large tin drum. I was quite pleased, too, until I thought what Tony’s mother would say when she saw it. Nobody would get any sleep for weeks! I led Tony away quickly, saying that the drum was too expensive. If that was how I felt, Tony replied jokingly, then I could buy him the big model railway in the shop window. Now that was really expensive, so I quickly changed the subject.

Tony asked for permission to go off on his own and I made the most of my opportunity to sit down and rest my aching feet. Fifteen minutes passed but there was still no sign of Tony. I began to get worried and got up to look for him. I asked a young lady if she had seen a little boy in a grey suit. She looked about her helplessly and pointed out that there were so many little boys in grey suits. I was just beginning to despair, when I saw a strange figure dressed in peculiar purple clothes. The figure was wearing a false beard and had a cave man’s axe in one hand, and a space gun in the other. It was, of course, Tony, who informed me at once that he was the first cave man to fly into space.