I suspect there is something wrong with your network connection if downloads are taking that long (it may also be a problem with your network provider and/or their path to the Fritzing site). On a slowish (1 megabit) ADSL line here in Canada it just said the download would take 8 minutes. While I too am not a Mac user, there have been some problems with github on some of the older Macs blocking parts updates, but that doesn't seem to be the problem here. It sort of sounds like a permissions problem (from the "Can't read resistor.svg" message), are you perhaps using a different user ID than the one that Fritzing was installed under? Can you display the ownership and permissions (I don't know how to do that with the gui on a Mac but ls -l from a command prompt will display the ownership and permissions of files) on the original file and then on the file that you replace? That should provide a clue to the problem I expect. Windows is even better as both Steelgoose and I are Windows types.
Peter