A First Officer questions who is legally responsible for a deferral when Maintenance Control directs a Contract Mechanic who physically completes and signs for a repair on an A320. Said Contract mechanic is reportedly untrained on the aircraft.

Narrative

I have noticed an alarming trend with Contract Maintenance at some non-hub airports. This trend was perfectly illustrated this morning on flight from ZZZ-ZZZ1. When we arrived at the aircraft this morning the temperature was -8C (approximately 17 degrees Fahrenheit). The aircraft had been parked in a remote parking area overnight with no heat. The Operations agent reported that the APU had been started approximately 1.5 hours prior to our arrival. As I started the Cockpit setup; I noticed an ECAM: Vent Avncs Sys Fault. The Captain and I reviewed and complied with the Flight Manual Fault Reset procedure. The ECAM message remained. We sent an Maintenance Report and advised Maintenance Control who responded with three Circuit Breakers to reset. This didn't correct the problem either. Maintenance Control then dispatched Contract Maintenance.Contract Maintenance arrived and asked what the problem was. After explaining the problem and our attempts to reset the system; the Contract Mechanic called Maintenance Control on his cell phone and we remained in the Cockpit while Maintenance Control ran the Mechanic through several system checks. Then came the MEL card (procedures) over the ACARS printer. We then watched as the Mechanic went through the deferral procedure step by step. The Mechanic had Maintenance Control stay on the phone and coach him through the entire procedure. The Mechanic asked us several times where switches were and whether indications were normal or not. During the 25 to 35 minutes that the Mechanic worked through the deferral steps; the horn in the Nose gear area that indicates 'No' air circulation through the Avionics bay; was sounding. We brought this to the attention of the Mechanic several times. The deferral directed the Ventilation Extract fan to be selected to Override. The Mechanic; without direction from the MEL card or Maintenance Control; decided to also place the Blower switch to Override. After reading the MEL card and noticing the discrepancy; I mentioned it to the Mechanic. He responded that he was trying to get airflow over the Avionics. Returning the Blower switch to Auto eventually silenced the horn. The complete lack of Airbus systems knowledge demonstrated by the Contract Mechanic was disturbing to the Flight Crew. The Mechanic couldn't answer general questions about the system; let alone the specific questions that need to be answered so that we could understand how the system would function in its deferred condition. This begs the question; who is legally responsible for the deferral? If the Mechanic who physically completes the repair; doesn't know why he is doing what he is doing; how can he sign for the repair? If it is Maintenance Control who is signing for the repair; how is that legal when Maintenance Control is relying on an untrained (in Airbus) Mechanic's word that the steps set out in the MEL have been correctly accomplished?