And they said it couldn't be done:
This worked for me, and performance is fine (probably because I have a relatively new Macbook). I can play I:R again, though I obviously won't be paying for DLC when I know it could break again in the next patch and they won't even try to fix it.
I found a workaround on my MacBook Pro 2015.
Before starting a new campaign go to settings from the main menu and turn map objects OFF.
This stopped the game from crashing and I was able to play...albeit very slowly.
This worked for me, and performance is fine (probably because I have a relatively new Macbook). I can play I:R again, though I obviously won't be paying for DLC when I know it could break again in the next patch and they won't even try to fix it.