On a usability front I echo that the "␣" is definitely confusing, especially next to a ",".
While it might sound like this should make your samples very different from others, it could actually act the other way and confuse the network. Hopefully, this will be improved in the next version of it, which I'll start training right after this data collection sprint.
You could use https://developer.mozilla.org/en-US/docs/Web/API/KeyboardEve... to take advantage of keyboard layout information to more easily classify a user. E.g. if a German user is typing in English their key will register as "KeyY" but their key will register as "z". Even if the neural net just picks up on "non qwerty = weird" I think it would significantly help in it's ability to classify these corner case users.
I look forward to seeing how future versions fare!