lets be clear , you are only allowed ot use assets you purchased inside the unreal engine, you cant take htem out and put them in untiy without a new liscense also if you have anyone work on stuff thats fine as long as its brought back into the engine under you , as in your project and then you and only you get rights to sell if its a game…they if they wanted to need to buy said assets if they wanted rights to sell and youd have to agree between you about selling rights of co made items
also im sure some kinda lawyer then or written or email agreement needs ot be spelled out.
pretty sure this is how you can do all you want with a few friends that might pull a texture out into say 3rd party software and bring back into the engine. ( note if 3rd party software prohibits you doing this then you cant use it )