Upon further thought, I might overstated the case of the editor being stripped out a bit. I can now contemplate a scenario where a good amount of editor code could still remain the EXE, while also consistent with what I observed in my previous investigation.
Specifically, we could imagine the conditional compilation to be more limited in scope, such that it does, for example, exclude the code that calls the Windows APIs to load the editor-related dialogs, without also excluding the bulk of code that handles the dialog itself.
Basically in Windows, when you create a dialog, you specify the template (which are the resources in the EXE) along with a pointer to a "dialog procedure", a handler function to handle all things related to the dialog. The dialog procedure is the code, the programming that handles any initial setup that should occur when the dialog is being loaded (eg. populate certain fields with the correct value to display), and most importantly, handles any responses to important user actions like clicking on buttons in the dialog or similar. The template is just the data describing layout of elements in the dialog, there is no code associated with it.
My previous investigation concludes that there are no Windows API calls to create dialog for any of the dialog resources other than "About". However, it is possible that the dialog procedure itself (ie. code the handles how to set up and response to user actions as related to the specific dialog) still remains un-excluded in the EXE, and that will constitute a decent amount of code since it is the bridge between the user interacting with the controls in the dialog, and the program actually responding and updating the level.
So it's theoretically possible there may be more editor code left in the EXE than I first envisioned. But even if that were the case (and that's still an "if"), it will still be very tricky to unlock--since there is no easy way to search for, say, Windows API calls to show a dialog (I tried that and only found "About"), much more work would be needed to figure what what parts of the EXE handles what things, in order to have a hope of finding the specific places where it would've shown an editor-specific dialog but the code to do so currently stripped out. And then you'd still need to work out all the missing code that needs to be re-inserted to make things work again. In short, even if it turns out maybe you don't have to rewrite the entire editor, there is still probably quite a bit of patching involved in this scenario where there's more editor code left in the EXE than I envisioned. It's not so much "unlocking" as it is "restoring" whatever's missing and excluded from the EXE. It might not be all of the editor as I originally concluded few years back, but could still be a pretty non-negligible amount of excluded code.