Baseband > 2.0.1700.48 won't work without proximity and light sensor working?
I have good news.  My tech put the sensor card back in my Nexus 4, reversing the rubber grommet that separates the sensors from the front glass.  The promity and light sensors then both worked, booting from 4.2.2.

I then used the stock image of 5.1 from https://developers.google.com/android/nexus/images to update the phone to the latest L release.

Everything works.  I take this as good evidence in favor the hypothesis that the absence of the sensor board was causing the boot loops described here and in the earlier article.