We would like to give players access to our custom Editor to create new maps. The legal page states that this is not allowed, unless the player has access to UE4 him/herself.
Is it allowed to provide our custom Editor as a separate package after manual verification that they have access to UE4? If yes, what would be the best way to distribute it? (marketplace?).
For a custom editor general distribution, the marketplace or GitHub are the way to go. Right now, the easiest way is for you to create fork of UE4 off the UnrealEngine network in GitHub, but your custom editor there, and tell your UE4 licensee engine customers to grab it there.