Refrigerant leak in older Maytag Plus MTB2156BEW?

[MomOfTwo]

The freezer has had lots of ice for a while now, and food in the back of the fridge was also freezing. I assumed this was due to poor door seals and I was intending to replace them. Then on Friday all the ice in the freezer started to melt. The condenser fan has good air flow but the air is not warm. The coils are a little dusty but not as bad as I expected considering they haven’t been cleaned in 25 years. The compressor feels warm to the touch. It seems like it’s vibrating a little, but it’s hard to tell since the fan is also vibrating. When I touch the outside jacket of the coil to suppress the vibration, the condenser vibrates less as well. The evaporator fan seems to be working fine as well. The refrigerator has been running continuously except when in defrost. On Saturday night (before total thaw) I noticed that the refrigerator stopped running and the back of the freezer felt warm to the touch, suggesting that the heater was on. By the time I got the back of the freezer compartment opened up there was no ice on the evaporator coil and it wasn’t even cold.  I used a screwdriver to engage the defrost timer and although the refrigerator stopped running the heater did not turn on, perhaps because the thermostat did not call for heat due to the warm compartment? I cannot find anything visually wrong except that the clips that hold the heater to the bottom of the evaporator coil are rusted and the heater dropped down. I have had the refrigerator running for a couple of hours now and there is no frost on the coil at all, only a little condensation on one of the lines. When I open the freezer compartment I can hear a faint noise. I’m not sure if it is vibration or refrigerant running through the coil. My only guess is a refrigerant leak, but of course I hope I’m wrong. I looked at the flow chart on this website as well and it seems like that could be the cause. What could cause a refrigerator that has been sitting in the same spot for 25 years to develop a leak out of the blue?

I’m also curious about these copper lines. They are different from say a mini-split heat pump and not what I expected. There are lines in the back that go up into the case and I assumed that they went to the evaporator coil but that doesn’t seem to be the case. I have no idea where they go. Instead, it seems like one line with a thin line running along side it goes to the evaporator coil. Very different from what I expected.