I’ve moved your post from the end of an unrelated thread to a new one, Stéphane, so that we can discuss the issues you raise directly.
For rehearsal marks, please see:
To move through a video frame by frame, hold the Command key (Mac) or Ctrl key (Windows) and use - and + on the numeric keypad, or alternatively F7 and F9. You can also move to any position in the video by showing the Key Editor and clicking in its ruler to move the playhead to that position.
It’s not clear what problem you’re having with the list of shortcuts. You can view all the current keyboard shortcuts via the Help menu, which shows a useful visualisation of the shortcuts on a keyboard. There is a logic and pattern to the way we have determined Dorico’s shortcuts, and they will make sense to you as you become more familiar with the software.
In order for us to investigate the problem you describe with inputting time signatures, please provide an example project that reproduces the issue. Some guidelines for how we can help you most efficiently: