Judges will rank games based on
gameplay - mostly if game is fun and if it feels complete
graphics - mostly if assets has same style
UI + onboarding - if game is understandable, smoothness of the process of learning and mastering it
bonus points - anything that hits judges taste and he wants to rate it, it might be an easter egg, some craziness, well used physics mechanism or genre
You will get majority of points for first three mentioned. Bonus points is just a minority. Thats all I can publish, hope it helps.
You can only update the score. Leaderboards are automated, yes. There is no way to change it directly or indirectly.
Plugin supports "Full screen controller" only. No you must not use those buttons and you are not expected to use them. In case you need buttons, make your own