No need to touch the iPhone at all.
I can activate the hotspot from the iPad even with the iPhone set to "Personal Hotspot > Off" without touching the iPhone. On the iPad, I just go into the wifi list in Settings, and above the list of Wi-Fi Networks is a Personal Hotspot list *it shows just my iPhone, together with the iPhone's signal strength and battery status. I just tap it and turns the Personal Hotspot on my iPhone on (which disconnects the iPhone from the wifi so it shares only the cellular connection).
Shortly after I turn wifi off on the iPad, or select another wifi network, the Personal Hotspot setting on the iPhone goes back to off and the iPhone reconnects to the usual wifi as well.