There actually is a system that shows what numbers are missing, but I need to make it darker and more noticeable (it's the colored background for each number).
"Can't erase placed numbers" Like the numbers you put down? You can erase them by hitting their number again, but I'll add in a backspace or clear option as well.
"Unclear what tile you have selecte at the moment if you're selectin on of the "wrong" numbers you've put down." Hm, good point. I'l look into that.
"No exit button." From the game or the main menu? In either case it's 'Esc', but the game will probably benefit from a GUI button as well.
Thanks for the feedback!