Ah ha! You are exactly right. I just tested it. I enabled cellular data, then turned the Home action off and on. It disabled cellular data. I left the house and walked down the block. The WiFi connection held on like a pitbull, but eventually it disconnected and cellular data turned on. On the way back, I was on my driveway before the Home action triggered, turning off cellular data and connecting via WiFi.
The one remaining nit is that WiFi needs to be on for this to work, and WiFi keeps telling me about other systems in the area I might want to connect to. Still needs some work, but thank you for the knowledge.
[UPDATE: Nit fixed. Just turn off the three check boxes "Notify me" (under WiFi Autostart Reminders), "Open Network" and "Secure Network" (under Network notification) and the searching and nagging goes away.]