Display plugin entityreference is not available
So - after getting another pair of eyes on this the issue is resolved. Somehow, the entityreference module was only partially installed and was missing the views directory (along with a couple other things) but was otherwise fully functional outside of the issue as described above. 

Not sure how the module would be working for other purposes, not throw any errors at all without those directories but that was the case. 

Thanks to Matt Glaman for the help tracking down the problem.