Regarding your first question, the behavior that you encounter is caused by a malfunction on your system. You shouldn't have this problem. To fix it, try to update your DirectX to a newer version. Also, it seems that the game is not compatible with Windows 8, so if you are using this OS, try running the game with compatibility features ON. To do this, right click on the game's icon > Properties > Compatibility tab and select Run program in compatibility with Windows XP.
The frequency of presented letters was set up by the developer when the game was created. This option can't be changed by regular users. In this case, there is nothing that you can do.