Hmm, I wish you've found this earlier as it basically collaborates some findings I've had with what I've done so far. That would've saved me quite some time and give more confidence in some of my unreported findings.
One thing I did not mention is the same conclusion noted on the website, that I too can only find code displaying the About box dialog. Basically there are only a small set of functions from Windows that consume dialog box resources, and by doing a search to track all places where the code is calling those functions, I only found the one place that is hard-coded to the About box dialog resource. The only reason I held back on reporting this is because theoretically there may be other more indirect ways that the Windows function call may happen that is not caught by the fairly simplistic searches I performed, however I do consider that unlikely, since there would be little reason for the editor code to be written to go through more roundabout ways to invoke the function compared to the expected normal way it did so with loading the About dialog (code for which I did find with my simple searches).
Also note that in the EXE has resource for the menu as well, but it contains no extra commands aside from the normal ones available. Even a search for extra strings that may suggest extra menu commands added dynamically to the menu yields nothing so far. This suggests there is no entry point from the menu to any of the dialogs, which is also consistent with finding no code whatsoever that seems to do anything with any of the dialog resources except About. Basically unless they for some reason decide to re-code the editor user interface without making use of those dialog resources, the more logical conclusion is that the editor functionalities have been stripped out. The final nail in the coffin is the observation that the error with the /EDIT@ is due to memory, but increasing the memory, while fixing the error abort, did not yield a working editor mode.
So sorry, I think all the information so far strongly suggests that the programming for editor functionality is probably removed in the game EXE, especially given that no snippets of code can be found by both me and that guy on that website, to load any of the dialog resources except About. Since the resources go through a different compiler than the code, it is not unexpected that they may have set things up to strip out the code for the editor during compilation, but forgot to do so (or did not set it up correctly) to strip out the dialog resources.
If the code are indeed removed from EXE, then there is no easy way to "make these windows usable again" short of basically rewriting the editor yourself and then shoehorn it back into the EXE via hacking. At that point you might as well just write your own separate editor program. I will grant you that it is difficult to rule out 100% that the editor code is truly removed without going through a detailed and time-consuming disassembly of the game (outside of the quick tricks we've already tried), but with all the evidence thus far pointing more towards the possibility of no editor code left in the EXE, and the game being relatively unpopular, I don't feel motivated to do more involved investigations that seems likely to yield negative results anyway.
