How I understand it is that his condition is not on activation but rather on change. So let's say he currently has cannon, but then Yuri thinks his ability is telepathy, that means his ability becomes telepathy and he can use telepathy whenever he wants... Until someone else believes his ability is something else and he selects that person as a target so his ability changes. It's possible his last used ability was the gun hand one or he told Yuri to believe that so he could change it at some point after the Touya fight. And that means he can always use Gun Hand. As far as I remember he can select the target even if he's not exactly in front of them (as proven by Kurisaki's grandma during his fight with Touya). Now the only problem with this is that the beginning of the series seems to go against this idea. With Akira not being able to summon the cannon until he aimed at Kirisaki during the second trial. Honestly idk.