5ng/ml means 5 nanograms per milliliter. It's a method of indication of the blood concentration.

All it indicates that fentanyl was already metabolized.

I doubt it can be added up but don't quote me on that.

To be perfectly honest, if you want to argue if he really OD'd or not you can't argue it only on toxicology report. He was an addict, had a high tolerance for the substance, so there's that - someone could argue that it wasn't enough for him. The only kinda sure way would be for coroner to show there were no indications of physical damage etc.