Can confirm Mod Editor DOES work without Galaxy, however, it requires a "trick" on the user's end. This is thanks to another user, Gersen, and was not at all obvious to me.
To access Mod Editor offline, no Galaxy, it should be accessible from the game tab in your Start Menu once you install the game. However, I do not use the start menu and will go into the alternative way:
1. First, install the game using offline installer.
2. Go into the folder where you installed the game; this will be where you see various files and there is a "Launch RealPolitiks II" shortcut.
3. Do not do anything with that "Launch RealPolitiks II" shortcut but rather, look right below it and see "RP2.exe."
4. Right-click RP2.exe and select "Create shortcut."
5. Now, right-click on your newly created shortcut and select "Properties," it should automatically be on the "Shortcut" tab and in the "Target" field.
6. Go to the end of the "Target" field, past the last quotation mark, PRESS SPACE BAR ONCE, then type -ModEditor, then click apply.
7. Now, double-left-click this modified RP2.exe shortcut to open it, intro videos will play, but it will take you to Mod Editor rather than the normal game main menu.
As this game just received some sort of update, may I add it would be nice if there was an easier way to access the Mod Editor without the user having to do these sort of tricks...as far as I can tell the Mod Editor is not available within the installation folder itself without this trick, but correct me if I am wrong.