Here is a question about one thing that I fail to notice ingame - How can one delete the city without destroying the terrain? Meaning once the city is deleted, the land is as untouched as it was before.

If I recall, there isn't really a way to 'restore' land. The closest thing to that would be wiping the city in the god menu, and making adjustments from there.

Copy the region from the GOG/SimCity4Delux folder (where your 'templates are) and paste it onto the correct region from your Documents/SimCity4 folder (where your actual saves are). Obviously you will need to 'replace' all the files.

In god mode there is the "obliterate" button, that deletes the city, but all changes you made in the terrain since you started the city are kept.