How do I update LTspice IV for macOS?
LTSpice for macOS has finally been updated to LTspice XVII as of October 25th, 2018.  I have verified that it has resolved the model update bug, as well as most other noticeable issues that had cropped up in the last release.  

So the updated answer to this question is to simply download the latest version - it fixes the model update crash and many other issues as well.  Even the icons have been tweaked.  

The new URL (since Linear Technology has since become part of Analog Devices) to download the latest LTSpice version (I would be surprised if the self-update function worked, given the change) is here.