1. What can it do to keyboards that don't suport more than one key press at a time.
Never heard of one. 2. Is it still bad to use a combination of the mane part of the keyboard with the arrow keys, or are the arrow keys designed to be used this way?
Irrelevant. All keys are equal (but just have different scan codes). 3. Is there a way to detect if a keyboard can handle multiple keypresses? Perhaps I could make a second system for those keyboards.
It's always a good idea to let the user define their own keys if they wish...