When the outdoor temperature is lower than the dew point of indoor temp, duct insulation is needed. Indoor relative humidity is needed to calculate the dew point, or you may estimate one, say 65%.
Your example 1 needs duct insulation, and example 2 does not.
I don't think it's a good idea to...
When the air leaves AHU, it will be cooled or heated to a constant temp (12.8°C or 55°F as you said). The airflow of the VAV boxes should be designed based on 55°F - 57°F inlet temperature.
Assume what will happen without duct insulation: supply air temperature might be heated to 65°F in...
