Dude, a sucerity guard caught you and did what he told you? Even if I was trapped in a stall of a bathroom, inside a movie theater. I'd tell that guy to fuck off, if he really was a stoner he wouldn't do a damn thing to you. Just be like, "What are you going to do? Call the cops? I'll walk away and be long gone before they get here." I've never said that to a sucerity guard, but it's what I'd say if I was in your situation.
As far as the rest. I would be extremely embarassed if some little kids dad claimed I was masturbating in the bathroom of a coffee shop. I would run out of there cherry red in the face most likely. I envy you for how you handled that whole deal.