Can we use assets that were given away during the monthly giveaways but have since become paid? Or just the permentantly free assets? 

You can submit works created using paid tools. You just cannot submit source with paid assets you don't have the right to distribute.

I hope so since they are free, but would like to see an official response :)  Not sure I will use the characters but the animations are nice and easily accessible :) Link for reference: https://www.mixamo.com/#/?page=1&type=Character

You may only use assets that anyone can currently acquire for free. RPGMaker-included assets are not free - you have to buy RPG Maker to get those assets. You may use assets that are free to use for non-commercial purposes as long as everyone is also able to get them for free right now without buying anything.

The majority of our participants are English-speaking users. You don't necessarily have to submit a game with many words if you don't feel comfortable, but I believe most of us won't be too harsh on your game's grammar as long as you're not making a story-heavy game.

You may use the free version. It's also available on the Unity Asset Store in both free and paid versions if you want to download from there.