Freezer working fine, but refrigerator not getting cold?

Sharp SJ-42E-WH self-defrosting fridge.

Having read a lot of your related answers to my previous question, and in anticipation of some of your suggestions, I carried out the following:-

1. I again removed the plastic housing contaning the evaporator fan and also the plastic plate behind it to expose the evaporator
2. Using a torch and mirror, I checked for ice build up at the bottom of the evaporator - there is none
3. Whether or not this has any bearing on my problem, I do think that the drainage pipe is blocked however, as the tray at the bottom of the fridge is bone-dry and full of dust
4. There doesn't seem to be any way to expose the inlet of that drainage pipe to unblock it though - the big plate at the back of the fridge (almost the full height of the fridge) appears to be spot-welded and despite having removed all the screws I could see, it won't budge 1mm. Would it be acceptable to pour boiling water down next to the evaporator from inside the freezer compartment? If that doesn't unblock it though, there is very limited space to be able to mop that water up from the top again.

Getting back to the original question - I still think it is air-flow related. In order for the cold air to blow down into the fridge below, the air inside the fridge obviously needs to be displaced and I assume this is achieved via the interior grill/vent in the roof of the fridge just below the top of the door. There appear to be two channels that would take the warmer air from the fridge into the freezer and back towards the evaporator which is located just below the top of the fridge door.

I held a blower vac to that vent, expecting the air to come blasting through into the freezer, but could not detect it at all. Because of the moulded construction of the freezer compartment, it's impossible for me to see if those ducts are blocked.

I'm trying to provide you with as much info. as I can, but forgive me if I'm confusing the issue. Maybe I'm clutching at straws, but I'm starting to run out of ideas as to what the problem might be.