You would remove the inside back wall of the freezer and you should have access to the drain. Have you already done this and not been able to get to that drain?
The "heater assembly" is the evaporator and not shown on this diagram. You may have to parital disconnect it. However, you need to be extra careful if you do so. If you damage any of the tubing you will have a sealed system leak and then you will be looking for a new refrigerator.
I've not seen a drain that was accessed from the outside back wall. So, I would only work from the inside. If the hot water and turkey baster procedure is not working for you, have you tried snaking a 1/4" flexible plastic tube down thru it?
Also, there is heating kit that be purchased to keep the drain from freezing if that is your problem (and not crud build up).
I'm sorry that you are having such difficulty. I haven't seen this serious a problem with any defrost drain before.
The web site shows that you have paid. So, you needn't be concern with that. If you receive any "reminders" just ignore them.
Good luck. Let me know how you make out because this an odd one I would like to know what the resolution is. You can post back to this question.