Most of the time, when you buy an asset they give you the license to use it however you want, you don’t need to credit them with it. To make sure, you’d have to check the user agreement from the place you bought the assets from.
As for other copyrights, you automatically own copyright on something you create without having to do anything–at least that’s how it is in the U.S.
That doesn’t necessarily stop someone from doing stuff but if you find someone doing something then you have legal rights if you want to pursue it.
For the donations stuff, the only thing is that you have to keep in mind that if someone is giving you money in return for access to the game then you will owe royalty to Epic on that money. If you don’t give them anything for the game in return then you don’t owe royalty.