Wouldn't CR-props get rid of the critical engine on a twin, for one thing? (downgoing blade produces more thrust than upgoing, hence on a twin with two 'regular' props one engines' thrustline is offset closer to the centre than the other, etc, etc)
Yes, they would. But twin does not need critical engine anyway - you can have two conventional contrarotating props, and no critical engine.
But if you have two contrarotating props which are not coaxial, then you are still shedding propwash and wasting energy on swirl. The point of coaxial contrarotating props is that they are eliminating the swirl.