The Demaree method seems unnecessarily complicated to me. A hive that is making preparations to swarm will almost certainly swarm. Like you said: sooner or later you will miss a queen cell. You can either do an artificial swarm, or make up 2 Nucs for sale. For an artificial swarm, find the queen and remove her to another box along with a couple of frames of honey, bot no brood. Also shake a great deal of bees into the "swarm" hive. A lot of them will drift back. The bees will think they have swarmed and raise a new queen. To make up a couple of Nucs I just buy queens and give them 1 frame of honey, 1 frame of pollen, and 3 frames of brood with bees from the parent hive. To me spending $25 on a queen and having a $150 Nuc to sell in a few days is sensible.