Why can't you reorder global variables list inside editor?

As the title says. Does the order of global variables have something to do with how variables are implemented in the engine?

Second question: I have an idea to reorder global variables manually by opening the json file and doing some copy-paste action. Has anyone tried this? Will it be safe for the project?

Yes, it will work. I suggest you make a backup of the game json before you edit it, and keep to the JSON structure. Other than that, you should be good…