Freezer cold, but Warm fridge side - not cooling

Most people think the problem of a cold freezer and warm fridge is with the flow of cold air INTO the fridge, which it can be with a FAN or DAMPER problem; but more often than not, it's a defrost problem:

When a (SxS or top/bottom) refrigerator is working, a small amount of cold air flows from the freezer fan into the fridge and then CIRCULATES BACK from the fridge into the freezer. Air goes from the freezer fan (inside back panel) through a hole/conduit between the freezer and fridge, then through a damper (controlled opening) and into the fridge. Then it circulates back from the fridge through a vent/flap (not close to the fridge air inlet), then through a hold/conduit leading to the evaporator (freezer inside back panel), then over the evaporator coils and into the fan inlet.

No air can flow from the freezer into the fridge unless it has a path to get back to the fan inside the freezer. If the return air flow path is blocked, then the fan simply can't push air into the fridge while the freezer door is closed because if it did, it would create a vacuum in the freezer, which is not possible because of the large vents in the freezer - i.e. the vacuum would then try to suck the air from the fridge back into the freezer. So even though the fan may still be recirculating air hard inside the freezer, it won't blow into the fridge until and unless there's an air path from the fridge back to the fan inlet. If the problem is with a blockage in the air flow path from the fridge into the freezer (as opposed to a problem with the air flow path from the freezer into the fridge), then there will be no air flow while the freezer door is closed. Opening the freezer door suddenly makes air available to the fan inlet from somewhere other than the fridge, which is why air suddenly is able to flow into the fridge. 

So the easiest first test of a warm fridge side is to check the air flow into the fridge side with and without the freezer door open. If the air flow into the fridge side is good with the freezer door open and bad without the freezer door open, then the return air flow from the fridge into the freezer MUST be the problem! You now only need to look at the return air path for blockages, because you know that there ARE NO BLOCKAGES (or damper problems) in the air path from the freezer fan into the fridge. 

Possible blockages in the return air path from fridge to freezer really only include the return vent/flap which starts inside the fridge plus the path through the evaporator. Clearly my problem is I have a blockage in either the return vent or at the evaporator coils. The return vent's a pretty simple flap that either moves freely or not. It can be propped open if need be to test things. Ice in the return line could be a problem of course. But I see now why warm fridges and cold freezers are most often caused by defrost problems. Air flow 101.