Thanks so much! This is great feedback!
For mode switching, perhaps dimming the text in the inactive area would add a little more clarity, or maybe making the border flash. I'll have a play and see what looks good. I love the idea of a quick flash of colour on an incorrect answer too.
I'll have a think regarding difficulty - I certainly don't think it hurts the game to give the player options here, my current leaning is potentially adding a slider to allow the player to amend the speed of the enemies to a level that suits them.
Cheers for the thoughts, really appreciate it :)