I believe the open-source of SMILE (super metroid) is available to look at. That will help at least me understand game-specific editing a little more. Where did you go to understand how to deal with Visual Basic and C++ GUI handling, such as how to attach a function to a button, open a window, close a window, etc.?
I was able to do some minor tweaking to the level design, such as change the colors of the enemies using offset values which was kind of neat. I remember trying to use the palette or tile editor before and grabbing my hair. My ultimate goal when trying to get Lucciano to release the source was to create a super user-friendly game editor. Even possibly being able to just drag and drop enemies, or palettes/tiles to change the design of the level. Only messing around for just an hour I was able to completely screw up one scene and then completely undo my mistakes manually. Just adding an Undo button, probably wouldn't be too difficult, you see it even in Windows paint, probably wouldn't take too long once I figure out the libraries.
I agree it really is a shame that you didn't comment the code (#1 rule in programming). However, I don't want to criticize you too much because I'm pretty rookie when it comes to dealing with the GUI, and I majored in computer science, yet focused on mainly web programming, not game or graphic design. The x2/x3 and internal emulation was pretty neat. Again if you could point me to some free guide online on how to program the GUI in C++ that would be great. I have VB 2015.
Just a side note. MM has mainly been ignored in the gaming community, for more popular games, yet even super metroid snes had major overhauls. I think it didn't become popular on the SNES because it completely avoided save games, and then on the N64 it looked really ugly. They haven't even had any recent remakes of the games, despite releasing some collections for the ps3 and the wii. Nintendo has mainly dumped MM, possibly because Capcom cross-platformed onto the Playstation quickly who knows. I've seen many rom hacks for the NES I'm really surprised only a few people have tried tweaking the 16-bit version. Once this project becomes an amazing utility that is very user-friendly there could be several new hacks in the next 5 years or so. I just watched a trailer for Megaman X Corrupted earlier. The developers are making the game on Adobe Flash I believe. Ridiculous. The project has been in the works for a few years now. It may even get abandoned before completion without releasing any beta files.