Yes, the insulated one is the cool one. When you say "unless it was able to thaw out over a few days", were you running the AC during those few days? It will thaw out in a couple of hours, typically, but you usually need to turn it off first. You want to run the AC for 8-10 hours constantly and then check for freezing. The evaporator, in the area you describe as "pretty cool" should be cold. Unambiguously.

The evaporator might make a hiss sound because you could be hearing the refrigerant escaping through the orifice tube/expansion valve. You probably know this already, but the AC works exactly like the cold created by discharging a CO2 fire extinguisher. The rapid expansion and transition from liquid to vapor, because of a pressure drop, absorbs heat. In an air conditioner, they just collect all that gas and then re-compress it and condense it in a cycle. So you can hear the hissing of the refrigerant some times, which is the gas expanding out the nozzle.

I can't tell from your photos, but is the evaporator coated with a spider-webby looking grunge? If so, that is going to have a big affect on the cooling. I am pretty certain your system is not operating at capacity. There's only about 4 things it can be: dirty/frozen evaporator, dirty condenser, low refrigerant, malfunctioning/clogged expansion valve. Well, add two more: there are two fans in the system. Is your condenser fan working? If the coils are clean, the fans are running, and the compressor is working and it's not getting cold on the evaporator, you should have the refrigerant level checked.