I would actually go for the editor as part of the game's main capx, this way it can share exactly the same mechanics you implemented in your "gameplay" part, and would allow the preview/test part of the editor to always be up to date with the current implementation, and would still allow to have the editor separated from the main gameplay from the simple fact it would be in different layouts and accessed from a different way from a main menu in the game.
The common gameplay mechanic would be contained in a specific event sheet that you can include whether you are playing the game or testing a level.
A specific event sheet depending on whether you are editing/previewing or playing would be the including the common mechanic event sheet.
You could reuse a basic gameplay layout in order to play the level in game or in preview.
If feels like even though you still would be developing two applications, you could still maintain a single codebase and still be able to provide working/updated builds to your level designer as you go.
This way you could even decide to provide the level editor to your players when the game is over, or simply hide/delete that part in the final builds.
As Soldjah mentioned, that would require you to possibly use some external files/data structure for the data of levels to be kept and worked on.