24 hour update on my case. Nissan techs identified two problematic battery cells, one very depleted and one partially depleted. Nissan authorized replacement of both. They tell me that when they actually open up the battery pack they will do further troubleshooting to see if they can identify any cause for this, and if so, there could be justification for a full battery replacement (for example, if there was some sort of leakage). But right now it looks like cell replacement and they'll have me on my way.
If that's the case, and no more engine lights materialize during my next five days of the refund warranty, I will feel good about this purchase. The price seemed to be on the lower end of what one might expect for the model/year/mileage/battery capacity, and now it will have been fully inspected, cells repaired and still have 5 years of battery warranty. It's been a bit of a hassle, but I now know more about the essential system of this care than I will on any other used car, EV or ICE.
One thing I would say coming out of this is that Nissan would do well to create a specific sensor for this issue. Getting the turtle light seems like overkill. That and CarMax should certify some of their techs on the basics of EVs if they are going to sell them. No way the tech should have told me to ignore the tutle light. Thanks again everybody.