Hey Paul!
Yeah, I would indicate a playhead for each track using differing brightness levels. Perhaps a trig is half brightness and the playhead is full brightness.
Yes, probability controls whether the gate happens. I could also experiment with skipping the stage, though I worry that would be a bit uncontrollable.
Yes, the parameters are displayed. When multiple trigs are held, the behavior might be to indicate the range of values somehow. Pressing a parameter button would set the parameter for the multiple trigs. This is essentially an Elektron inspired UI (hence the trig terminology).