I know you probably have not gotten to this story as yet, Fran, but the first appearance of the rings was at the end of "The Red Stallion." At that point, Ellery has what I might call "a big emotion". His reaction to Colson even agreeing to a commitment ceremony in that story was great surprise, and for him, the symbols of commitment are very powerful. This story takes place a week after the end of "The Red Stallion" when Colson is laid up in bed in serious condition, so Ellery is pretty much in the same fragile state of mind since he first bought the rings. So he is very insecure right now and his asking twice is a reminder that Ellery still hasn't "gotten over" the fact that Colson agreed to an actual marriage event.
So no, it wasn't "all resolved in book 6" but Ellery has been showing his vulnerable side and his insecurity in flashes, and the culminating event of the ring ceremony shows just how much it means to him that Colson is willing to go through with it - and Ellery now, right afterward (only a few days) is still in that state of mind.