If you find any glitch, you should report it here. Also, you can type -toggle or -t while playing the first chapter of the campaign to lock or unlock the buttons for accessing each chapter.
You can type -d in any chapter to check the difficulty. If you don't want your difficulty to change every time you load the next map, and you are playing in the Classic client, you should check out this video:
If you are playing in the Reforged client, you need to hit "Quit" after every map. If you don't, the difficulty will reset to Normal when the next map loads.