Something is not right here...
From your rules:
3. Melon Gaming reserves the right to publish all games made by participants to the Facebook Instant Games platform and the platform chosen by Melon Gaming.
5. All uploads (code, assets, executables, etc.) are licensed under Melon gaming.
8. You must share any codes or assets used for creating the game, that you’re legally owned without breaking any other legal agreement.
9. The games must not feature any brand names or trade mark other than sponsor’s trademark.
So, as I understand it:
You want participants to make games for you, games that you'll use commercially, without paying them, except a few winners. And you obviously hope to get much more then prizes cost. And as I understand, you will publish ALL games, not only winners. And participants probably will have to pay for artists or something like that with their own money. You also grab all codes and other intellectual work for yourself, though you don't do anything to make them. Maybe a participant will invent some new approach and won't want to give it away just for a thank you? None of other known engine developers have such demands. And you even forbid participants to place their own trademarks or names, to show it were them who did it.
No right for the game made my me, no right to advertise myself in game made by me, no guaranteed pay, no part of revenue, no royalties, nothing.
Question arises - if someone can make a game that will bring profit, why he or she would ever bother with agreement like yours?