I am not a Mac user, but I believe the instructions for repairing through Origin are the same whether you use a Mac or a PC. Find the folder called SimCityUserData and delete it. Then right-click the SimCity icon in Origin and choose the repair option. The instructions for downloading the region to use in Offline Mode are also the same.
You could also try using the Origin reset tool. Many Mac users have reported that it helps with many issues in Origin games. Instructions are here: https://help.ea.com/en-us/help/origin/origin/how-to-reset-origin-on-mac/
If anything helps to recover your city, please let us know as it would be great to share with other users who have the problem in the future. 