I just wanted to say thanks for the helpful insights. On closer inspection, the evap is EXACTLY ikkö-ikki described, lots of tiny erupting volcanos. I decided to charge to see what I could achieve.
The odd thing is my low pressure gauge read -25 psi on the outlet when running. By adding 134a I was only able to get it up to about -15psi. The Subzero specs said it should run at 10-20 psi low pressure side. I did not get near to this and it was no longer pulling much gas. However, by this point I had a frosted evaporator and my 12 oz. can of 134a seemed about half gone. (The specs also said it holds 6 oz.) So, I decided to err on the side of caution. It cooled quikcly to preset temps.
Of course, I know from you guys this is a temporory solution not a fix.
At least I learned a little and know that the unit is otherwise good, not a failing compressor, fans or thermistors. So, if (when) it fails again I will invest in trying to have the evap replaced. Thanks again for the advice and education.