Hmm... on my a570 with scriptless remote the camera only half shoots, no matter how long I press the remote button. That can't be right can it?
With synchable remote enabled it works (although the button must be kept depressed long enough for autoexposure to finish I think), including this override situation you're describing (with or without manual focus, I tried M mode Tv=5 s with CHDK Tv override of 20 s.).
