Your return air vents are blocked. I've had your exact problem on numerous LG built bottom-freezer refrigerators. You were on the right track pouring hot water from the top down the vents but you weren't persistent enough. The air duct goes to a small opening behind the freezer evaporator fan so you can have 12" of ice blockage. You can unplug the refrigerator with the doors open for 48 hours OR use a hand-held steamer and it'll take 20-30 minutes.