There would be an alternative somewhat simpler method:
if all the fields are in a frame, and the buttons are outside the frame, then you can have an Exit frame event. Again here you cannot establish which button is getting the focus. So in the exit frame event you can set a variable to the name of the last control that was active (with PreviousControl). Then you only need enter events for the controls outside the frame and if it is the right button you ignore any checking, if it is a wrong button you still do the checking and go back to the control if required.