If it is a sealed system leak, how easy is this to positively diagnose, i.e. what is involved? Is it possible for a sealed system leak to run this way for a year without further degradation? It's been running this way for at least 12 months.
In the attached photo I have a picture of a leaky evaporator, not the frost pattern, but the little dimples in the coil that are usually obvious. A 15 year old SubZero is definitely worth paying a professional to replace the evaporator, it's not something a homeowner can do on his/her own.