Looked a little harder and it turns out I wasn't the only one suspecting the ADC of preventing the feather from going to sleep. Anyway, the reason and fix for it is way over my head. Something to do with floats and the FPU?

The good news is after some copy-pasting ...

   #if (__FPU_USED == 1)
  __set_FPSCR(__get_FPSCR() & ~(0x0000009F)); 
  (void) __get_FPSCR();
  NVIC_ClearPendingIRQ(FPU_IRQn);
  #endif
  uint32_t err_code = sd_app_evt_wait();

We got sleep back!  Same as before, looking at 2mA without any LED(s). Aaand I think I'm finally done here! If anybody goes and builds this feel free to post if you run into any issues or even better have suggestions for the build.