I'm a bit confused on what you're trying to accomplish. This is currently controlled by a checkbox in the Persona window.
Go to the Persona Window and click the checkbox next to InstaClick Ability. This is in the Persona window and NOT in the game Options windows. The tool tip for the checkbox says "Click abilities cast immediately by canceling curent cast and qued abilities."
As long as that button is checked, clicking a cure will cancel whatever you're currently doing and cast the cure instead. My player window add-on that allows you to cure yourself with potions uses this setting as well. Here's what the code means:
parent.parent.CancelSpells.Activated=parent.parent.parent.Persona.InstaClick
parent.parent.CancelSpells.Activated is the True/False setting that controls whether the currently casting spell should be cancelled or not.
parent.parent.parent.Persona navigates up through the tree of objects from the window you are currently clicking on, up three levels, and then down to the Persona window.
Persona.InstaClick is the checkbox I mentioned above in the Persona window.
So if that checkbox is checked, its value is True. If it's not checked, its value is false. So when it takes CancelSpells.Activated and assigns it the value of Persona.InstaClick, what it's doing is cancelling the currently casting spell if, and ONLY if, you have that checkbox checked. If it's not checked, it won't cancel it.
This is NOT to be confused with the game option that controls whether or not spells get queued up. The InstaClick setting in the Persona window, unless I'm wrong, ONLY controls whether the cure buttons, and the three programmable buttons in the raid window, cancel the currently casting spells. It does NOT mean that your spells will no longer queue like they do now.
Is this what you were trying to do or am I completely confused (highly possible)?